Experimental autoimmune orchitis (EAO) in mice has been induced by repeated injection of a mixture of syngeneic testis homogenate and Klebsiella O3 lipopolysaccharide (KO3 LPS) as a potent adjuvant. The antisera obtained from mice with EAO lesions defined several antigens with apparent molecular weights (MW) of 38,000 (38 kd), 86 kd, 100 kd, and greater than 200 kd by the immunoblotting method. These antigens were organ-specific and exclusively present on the acrosome of spermatozoa, suggesting that these acrosomal antigens were highly relevant to EAO. It was found that the antigen with a fairly high MW (greater than 200 kd) was expressed on spermatozoa from the epididymis. Furthermore, the acrosomal 86 kd antigen was predominantly expressed in the testis, while the 100 kd antigen was dominant in the spermatozoa from the epididymis. It was therefore suggested that the 86 kd and 100 kd antigens in the acrosome were differentially expressed on the process of maturation of spermatozoa.