Comments: As a research scientist for S.H.I.E.L.D., Bruce Banner looked to recreate the super soldier serum that turned Steve Rogers into Captain America. instead Banner accidentally created a formula that turned him into the uncontrollable green monstrosity known as the Hulk. Yet S.H.I.E.L.D. and Banner persisted until an important ingredient, a sample of Captain America's blood, helped advance the formula. Now, a step closer to perfecting the super soldier serum, this new mixture transforms Banner into a more intelligent, yet grey Hulk.
Source: Marvel Comics - The Ultimates, The Ultimates 2, Ultimate Hulk Vs. Wolverine