The antigenicity and cross-reactions of bovine seminal constituents were studied using complement fixation, agar-gel-diffusion, and immuno-electrophoresis. Bovine ejaculated spermatozoa, seminal plasma, and blood serum contained common antigens such that antisera produced with one also reacted with the other two. Spermatozoa per se were antigenic when injected intradermally with Freund''s adjuvant into rabbits. In addition, ejaculated spermatozoa were coated with seminal plasma proteins. At least some of these coating antigens were found on spermatozoa from the vas deferens and must therefore have been deposited before the contributions of the major accessory sex glands came in contact with the spermatozoa. Antisera prepared against epididymal spermatozoa reacted with epididymal and ejaculated spermatozoa but not with seminal plasma or blood serum. Ejaculated spermatozoa possessed at least seven antigens. At least five were shared with seminal plasma and at least one with blood serum. Absorption of anti-sperm immune sera with seminal plasma and blood serum revealed that epididymal and ejaculated spermatozoa shared at least three protein antigens which were not found in seminal plasma or blood serum.