Alpha-fetoprotein (AFP) was purified from pooled human cord serum to determine whether it would be similar to purified mouse AFP in its ability to be transformed into an antiestrogen by incubation with estradiol (E2). Greater purity was attained with a three-step purification procedure of chromatofocusing, Blue-Sepharose chromatography and immunoaffinity chromatography than with a two-step procedure of polyacrylamide gel electrophoresis followed by Blue-Sepharose chromatography. Nevertheless, both procedures rendered AFP in a form that was transformable by E2 to an antiestrogen, although the product of the three-step procedure afforded more consistent biological activity. Removal of albumin from AFP was crucial for transformation of AFP to an antiestrogen. Thus, human AFP is similar to mouse AFP in being transformed to an antiestrogen upon incubation with E2, even though there is only 66% structural homology between the two proteins, and human AFP lacks the high-affinity binding site for E2 present in the mouse AFP molecule.