The Super Soldier serum, originally created by Dr. Abraham Erskine in Captain America: The First Avenger, has a long and detailed history in the Marvel Cinematic Universe. Along with the Infinity Stones and the Chitauri technology from the battle of New York, it represents one of the primary in-universe reasons for human beings to possess super powers. The franchise has developed its continuity very carefully over the course of its run, and the serum has been a big narrative beneficiary of that.

Canonically, it has appeared in seven specific instances in the MCU (including Steve Rogers himself), though characters have referenced it more often than that. It will likely crop up again in subsequent projects. For instance, Black Widow’s powers, if she has any, have yet to be formally defined, and could easily derive from a serum variant. When taken together, these instances demonstrate how the MCU displays in-world consequences for using such scientific advancements, and how they don't always move in directions their creators intended. What follows is a list of the seven known uses of the serum, in chronological order.

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Steve Rogers and the Red Skull

For years, no one had yet managed to duplicate Abraham Erskine’s original serum, which created both the greatest and most terrible results in the MCU. When HYDRA murdered Erskine in Captain America: The First Avenger, part of the formula died with him. Before that happened, Steve Rogers became the first fully successful recipient of the serum, eliminating his plethora of health problems and granting him enhanced physical abilities.

More importantly, the serum had some important spiritual effects. As Erskine explained, “the serum amplifies everything that is inside, so good becomes great, bad becomes worse.” That helped Steve Rogers to become his best self, going on to lead the Avengers as Captain America. But it also resulted in the creation of the Red Skull when Nazi scientist Johann Schmidt injected himself with the serum. Schmidt’s “death” and Rogers’ hibernation meant that both the formula and its two recipients were lost for decades. In the interim, many variations were attempted.

Bucky Barnes

Armin Zola began experimenting on Bucky Barnes in 1943, after Barnes and other members of the future Howling Commandoes were taken prisoner by HYDRA. The experiments continued after Bucky’s apparent death in 1945, using Zola’s version of the serum and resulting in the creation of the Winter Soldier Program. It’s unclear how much of Bucky’s resulting instability came from the serum, as opposed to his brainwashing and subsequent abuse. Bucky, however, exhibited the same powers as Steve, and once Wakanda purged HYDRA’s influence from his brain, he became a hero in his own right. Zola was recruited by SHIELD as part of Operation Paperclip and secretly recreated HYDRA within the organization. That resulted in subsequent versions of the Super Soldier serum, both good and bad.

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Isaiah Bradley

Sometime after the war, the US government injected Isaiah Bradley and other soldiers with an unknown variant of the serum. Although not all of the others were stable, this resulted in another successful Super Soldier in Bradley. He was sent to Korea, where he confronted and defeated the Winter Soldier after rendering his metal arm useless. Despite his heroism, he was betrayed by his own government, who imprisoned him for 30 years and experimented on him. It’s unclear whether Zola orchestrated this as part of his stealth plans for HYDRA, or whether it was simple awfulness by the US government, but it left Bradley righteously aggrieved and unconvinced that a Black man could ever hold the mantle of Captain America. He retained his strength and physical abilities well into old age, as evinced in The Falcon and the Winter Soldier, Season 1, Episode 2, “The Star-Spangled Man.”

The Red Guardian

Introduced in Black Widow, Alexei Shostakov, aka Red Guardian, is the Russian counterpart to Captain America. Played by David Harbour (Stranger Things, Hellboy), Alexei is the sole product of Russia's own Super Soldier program -- indeed, he displays strength greater than Steve Rogers or Bucky Barnes -- although the details of his origin are a bit murky.

In the comics, Alexei was a famed Soviet fighter pilot chosen by Soviet Premier Nikita Khrushchev, who in the 1950s recognized that the U.S.S.R. needed an answer to Captain America. Faking his Alexei's death, the KGB trained him in secret, and equipped him with a magnetic throwing disc attached to his belt buckle. The big difference between the MCU and comics versions of Alexei is that the latter was merely a skilled combatant, and possessed no superhuman abilities. (Well, one of the big differences: In the comics, he was Natasha's husband rather than her adoptive father.)

The Winter Soldiers

The Other Winter Soldiers

Captain America: Civil War entailed a quintet of additional Winter Soldiers, developed by HYDRA in the early 1990s from serum samples held by Howard Stark. Barnes assassinated Stark and his wife, Maria, and took Stark’s samples to facilitate the creation of new Winter Soldiers. However, the new units were aggressive and unstable, turning on their HYDRA masters. They were subsequently placed in cryogenic stasis and all but abandoned in a Siberian facility. Steve Rogers and Tony Stark arrived there during Civil War, convinced that Baron Zemo intended to reactivate them. They found that he had killed the quintet instead.

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Bruce Banner and Emil Blonsky

Hulk fighting Abomination in The Incredible Hulk

The Hulk’s history in the MCU is a little rough, owing to the difficulties surrounding 2008’s The Incredible Hulk and Edward Norton’s subsequent replacement with Mark Ruffalo. But the details of his unfortunate experiment involve a variant of the Super Solder serum, which Thunderbolt Ross hoped to re-create. Banner employed gamma radiation in the experiment, and the “classic” Hulk was the result: triggered by rage, inhumanly strong and impervious to harm from anyone save Thanos. Once Banner went on the run, Ross recruited Emil Blonsky to pursue him. Blonsky ended up taking a compound made of Banner’s blood, transforming him into the Abomination. He possessed similar powers to Banner’s, as well as a thirst for violence.

The Centipede Serum

While its current canonical status is questionable, the Centipede Serum combined the Super Soldier serum, Gamma radiation and the Extremis formula from Iron Man 3 to create a new breed of superior warriors. It resulted in multiple users, most notably John Garrett who took it to avoid dying, and Mike Peterson, who was captured by HYDRA and augmented by cybernetics to become Deathlok. Early experiments with the serum resulted in disaster, and Garrett was unable to derive superpowers from it (it merely kept him alive). But others – dubbed Centipede Soldiers – proved more successful and served as adversaries to Agent Coulson and his team in Agents of SHIELD.

John Walker and the Flag Smashers

The Flag Smashers stole vials of the new Super Soldier serum from Dr. Nagel, an ex-HYDRA and CIA agent funded by the Power Broker in The Falcon and the Winter Soldier. Karli Morgenthau and the other Flag Smashers used the variant on themselves, gaining the requisite strength and physical effects. She intended to use additional vials to create more Super Soldiers for their ranks, but Baron Zemo destroyed most of them before she had the chance. The last vial fell into the hands of John Walker, who took it in a moment of agitation after being beaten by the non-powered Dora Milaje. The more personal effects described by Erskine may have been in evidence as well, with Karli growing more fanatical in her cause and Walker’s rage becoming homicidally extreme.

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