Mouse sperm were rapidly immobilized when exposed to rabbit antisera against rat calreticulin. The inhibition of sperm motility was concentration dependent at dilutions 1:50-350. The velocity of sperm did not change significantly as long as they were motile. Neither motility nor velocity of sperm was affected by adding sheep antisera to bovine calmodulin. The antisera to calreticulin also inhibited in vitro fertilization of mouse eggs. Using the avidin-biotin-peroxidase complex method, the antigen was found to be localized in the acrosome of sperm. The results indicate that calreticulin is present in the acrosome of mouse sperm and may play an important role in sperm motility.