Two isozymes of hexosaminidases (Hex) were purified from human seminal plasma and found to be homogeneous as revealed by immunoelectrophoresis and immunodiffusion experiments. Anti-Hex A antibodies were precipitated with isozyme B and vice versa. Though the precipitin arcs were equally stained for protein in these two antigen and antibody systems, enzyme activity was weakly stained in the reverse systems (ie, Anti-Hex A with Hex B and Anti-Hex B with Hex A). Thus, some common sequential antigenic determinants were indicated in two isozymes of Hex. On the other hand, loss of enzyme activity in the precipitate of reverse antigen-antibody complexes as compared to direct system (Anti-Hex A with Hex A and Anti-Hex B with Hex B) revealed that the two isozymes are present in different sequential and conformational states near the active sites. Moreover, antibodies of both forms of Hex cross-reacted with human sperm and reproductive and other vital organs such as kidney, liver, lung, pancreas, and spleen. Though monkey tissues also cross-reacted with human seminal plasma Hex-antisera, mouse, rat, and goat testis and epididymis extracts failed to show any cross-reaction. It is concluded that human Hex(s) are tissue nonspecific but appear to be specific to the human and the subhuman primates.