Administration of cyclophosphamide (5, 10, 20 and 25 mg/kg body weight) to male CD-1 mice 2 hours after subcutaneous implantation of a 5-bromo-2'-deoxyuridine (BrdU) pellet (55 mg) resulted in a dose-dependent increase in sister chromatid exchanges (SCE) in bone marrow cells. Treatment with cyclophosphamide (15 mg/kg body weight) at the time of BrdU implantation and 2, 6.5, and 13 hours post-BrdU implantation resulted in the induction of approximately 19 SCE/cell indicating that the bone marrow SCE response was independent of the time of administration. The time-dependent induction of SCE is qualitatively similar for CP and MMC both of which are bifunctional alkylating agents metabolically activated by oxidation and reduction, respectively, and suggests that these two compound may induce SCE by a similar mechanism.