When mouse spermatozoa were washed immediately upon release from the epididymis, preincubated for up to 120 min in PVA-containing, albumin-free medium and assessed for their ability to fertilize cumulus-intact eggs in vitro, they were poorly fertile in comparison with their unwashed counterparts in the same medium. Fertilizing ability could be significantly improved by introducing taurine or albumin or by washing a second time at the end of preincubation. The most effective treatment was provided by the continuous presence of low concentrations (0.05-0.1 mg/ml) of BSA, similar to the amount of albumin detected in the supernatants removed during washing. There was no evidence that acrosome loss was inhibited by washing; rather, it was enhanced by the removal of a surface component which inhibits the acrosome reaction. The presence of taurine did not further increase this response. Motility, reduced in washed suspensions, was improved by the presence of taurine or albumin and experimental results suggest that this was a major factor in the improvement of fertilizing ability after introduction of these compounds. Although taurine, hypotaurine and albumin were all found in the sperm washings and thus would be present in unwashed, fertile samples, low concentrations of albumin were able to maintain full fertilizing ability. Therefore, unlike hamster spermatozoa, mouse spermatozoa would not appear to have an obligatory requirement for a motility stimulating factor such as taurine.