Polymorphism of mouse sperm was investigated by analysis of immune sera generated in BALB/c female mice against sperm from 6 inbred strains. The immune sera were analyzed by immunofluorescence and Western blot techniques against sperm antigens from the 6 immunizing strains. Immunofluorescence revealed no differences in reactivity patterns or titers. However, several different reaction patterns were detected by Western blot technique which indicated that both the sperm extracts and the antisperm immune sera contained different components. Syngeneic (anti-BALB/c sperm) antisera showed far fewer reactive antibody species than allogeneic immune sera. The anti-BALB/c sera recognized an antigen of 23 kDa in sperm extracts from DBA/2J and C57BL/6 mice, and failed to react with an antigen of the same molecular weight when applied to sperm from A/J and 129/J mice, indicating antigenic differences between sperm from these inbred strains. Anti-C57BL/6 sera contained a unique antibody which reacted with an antigen of 80 kDa in all of the 6 sperm extracts, whereas others antisera did not detect this antigen. These findings indicate antigenic and immunogenic polymorphism in sperm from different inbred strains of mice.