An objective antiglobulin radioimmunoassay for the analysis of anti-spermatozoa antisera has been used to characterize more than 30 alloantisera. Each of these antisera was raised against spermatozoa from mice carrying T/t regin mutations. Whilst all of the antisera had considerable activity for mouse spermatozoa, none of them was found to contain antibodies specific for T/t region gene products. A detailed analysis of a single serum revealed that its activity is for antigens which are non-polymorphic, sperm-specific and species-specific. The failure in this study to detect antibodies specific for T/t region gene products is discussed.