The removal by centrifugation of epididymal contents from mouse spermatozoa had no deleterious effect on fertilization in vitro and, depending on the genotype of the gametes, was frequently associated with increased levels of fertility. Washing of the spermatozoa significantly improved the fertilization rate of F1 eggs with TO spermatozoa and of BALB/c eggs with BALB/c spermatozoa, but had no significant effect on F1 spermatozoa with F1 eggs.