Sera (vbs, n = 25) and seminal plasma (vsp, n = 21) from vasectomized men (n = 25) were analyzed for cross-reaction with lithium diiodosalicylate (LIS)-solubilized human sperm extract, protamine, and fertilization antigen (FA-1) with an enzyme-linked immunosorbent assay (ELISA). Among the vbs tested, 44% reacted with human sperm extract, 28% reacted with protamine, and 44% reacted with FA-1 for at least one class of antibodies (IgG, IgA, or IgM). In contrast to the sera, the seminal plasma showed minimal reactions. Neither the vbs nor vsp were found to contain immune complexes, indicating that the antibodies were present in free form. Vasectomized sera that reacted with FA-1 showed a significant (p less than 0.0001) inhibition of human sperm penetration of zona-free hamster ova. The immunoabsorption of FA-1-positive sera with purified FA-1 significantly increased the penetration rates. Affinity-purified human immunoglobulins reactive with FA-1 and not those reactive with protamine reduced sperm penetration rates. Thus, antibodies in vbs reactive with FA-1 are relevant to infertility, causing an inhibition of fertilization. These data will have clinical relevance for diagnosis and treatment of infertility after successful vasovasostomy.