Tales to Astonish #27 (January 1962)

"The Man in the Ant Hill" reads like a one-off monster-era shocker: a scientist named Henry Pym shrinks himself and gets hunted by giant ants. Kirby's cover plays it as pure suspense, with no costume and no hero name anywhere. Pym returned in costume as Ant-Man eight issues later, which is why collectors now treat this anthology cover as a Marvel Universe debut.