In many animal models, primary tumor removal produces increased proliferation of cells in metastatic foci. The present investigations using a murine mammary tumor were carried out to determine how a variation in the time interval between primary tumor removal and administration of a single dose of cyclophosphamide (CY) affected labeling indices of residual tumor cells, their growth, and animal survival. The CY (240 mg/kg) had a more favorable effect when given on the day of tumor removal than 3 days after, a time when the labeling index (LI) of metastases was at a peak. It was least effective if given at 7 days following primary tumor excision, when the LI had returned to the preoperative level. The greatest effect occurred when the CY was given prior to operation. It completely prevented the increase in LI resulting from tumor removal, more effectively suppressed the growth of residual tumor, and prolonged survival to a greater extent than was noted under any other circumstance. The interval between tumor removal and administration of a relatively small amount of CY (60 mg/kg) was critical. When given on the day of tumor removal, an increase in LI of the residual focus occurred which was greater than that occurring as a result of tumor removal. When given 3 days after tumor removal, the smaller dose was almost as effective in suppressing LI as was the larger. From a kinetic standpoint, there was no advantage in reducing the tumor burden prior to the use of chemotherapy. The tumor response in this model suggests that, for the most effective control of metastases, the largest tolerable dose of chemotherapy would best be used at the time of or before primary tumor removal. The results provide a biological rationale for the use of perioperative adjuvant chemotherapy.