Whereas there have been several recent reports of the induction of apoptosis by chemotherapy agents in cell culture systems, much less is known about the role of this mode of cell death in tumors treated in vivo. We therefore quantitated the proportion of apoptotic cells induced as a function of time and dose in two murine tumors treated with cyclophosphamide in vivo. The two tumors were a mammary adenocarcinoma, MCa-4, and an ovarian adenocarcinoma, OCa-1. The percent apoptosis was scored from stained histological sections of the tumors using a system based on the characteristic features of the apoptotic nuclei. The kinetics of apoptosis development were determined over a 5-day period following treatment of the mice with 200 mg/kg. The percent apoptosis peaked between 10-18 h in both tumors and then slowly declined to background levels by 5 days after treatment. The dose responses showed that even much lower doses, 25 mg/kg, could induce significant apoptosis and that the proportion of apoptotic cells plateaued at doses higher than 100 mg/kg. These results are compared and contrasted with our previous reports on apoptosis induction in these same tumors with ionizing radiation.