The effect of cis-diamminedichloroplatinum (cisplatin: CDDP) on mouse spermatozoa was evaluated quantitatively by means of equilibrium sedimentation in Percoll. CDDP was administered subcutane-ously at doses of 1, 3, and 10 mg/kg/week for 5 weeks. After different periods (1, 3, and 10 weeks) without CDDP, the quality of epididymal sperm was evaluated by sperm count, motility, morphology of sperm, and the apparent density of sperm. At 10 mg/kg dose, about 80% mortality occurred during the administration period. There were no sperm even 10 weeks after discontinuing CDDP. With the 3 mg/ kg dose, sperm count, motility, and normal morphology of sperm declined after 1 and 5 weeks, but recovered to the control level after 10 weeks. The sperm distribution profiles in the Percoll gradient were quite different among the groups. The control sperm showed two separated peaks in the gradient, whereas the peak of sperm at 1 and 3 mg/kg were shifted forward to lighter apparent density. CDDP causes a reduction in sperm apparent density and impairs semen quality in mice.