Studies of testicular cell killing in mice by several chemotherapeutic drugs have been performed to evaluate the harmful effects of oncolytic agents on reproduction. Seven drugs, Adriamycin, 1-β-d-arabinofuranosylcytosine, bleomycin, cyclophosphamide, hydroxyurea, vinblastine, and vincristine, given as single injections, were cytotoxic to differentiated spermatogonia. Adriamycin was also highly effective in killing stem cells. The other drugs produced little or no stem cell loss even at doses toxic to the animals. Negligible killing of spermatocytes and spermatids was noted at any dose level. The results demonstrated that oncolytic agents preferentially killed cells at specific stages of the spermatogenic pathway in mice at doses within the clinical range for humans.