A Graffi murine leukemia was utilized as a model system to investigate the effect of chemoimmunostimulation therapy. Subcutaneous inoculation of approximately 1.0 times 10(6) tumor cells resulted in a rapidly growing tumor at the site of inoculation and subsequent development of splenomegaly and lymphoadenopathy. All animals succumbed to the leukemia within 24 to 30 days. Treatment of diseased animals with two courses of cytoxan over a 2-week period resulted in a remission period of approximately 16 to 18 days before relapse and eventual death of approximately 70% of the drug-treated animals. A significant number of long-term survivors (50 to 83%) was obtained in groups of animals that received combined drug plus BCG or C. parvum therapy. In contrast, the administration of MER (a methanol-extracted residue of BCG) to animals in a drug-induced remission period was no more effective than drug alone. The protective effect afforded by BCG and C. parvum was dependent on the time interval between drug therapy and the administration of the immunostimulators. Treatment of leukemic animals with BCG, C. parvum, or MER alone proved ineffective as all mice died at approximately the same time as untreated control animals. No leukemic cells were observed in any of the histologically examined tissues taken from long-term survivors. The implication of these results for cancer therapy is discussed.