AbstractFisher rats were treated for 28 or 60 days to multiple exposures to the smoke of marijuana or marijuana placebo cigarettes. Primary, secondary and in some instances tertiary tumor implants were performed. Murine sarcoma 180 tumor cells (7.5×107) were implanted subcutaneously on day 1, 14 and 28 following initiation of smoke exposure (28 day studies) or on day 1, 14 after cessation of smoke exposure (60 day studies). Tumor areas were measured on alternate days beginning on the second or third day after implantation for 13 or 14 days. Exposure to both marijuana and placebo smoke for 28 days (6, 9 and 18 cigarettes per day) resulted in suppressed growth of secondary and tertiary implants. Administration of△9tetrahydrocannabinol (50 mg/kg, i.p., 20 days) failed to suppress the growth of primary and secondary tumors. This suggests that noncannabinoid constituents of the smoke may contribute to the suppression of tumor growth. Exposure of rats to 9, but not 4 or 6, marijuana or placebo cigarettes per day for 60 days suppressed the growth of primary but not secondary tumors. Thus, the effects of smoke exposure appear to be lost by two weeks after cessation of treatment. The possible existence of a non-cannabinoid immunostimulant in the smoke is discussed.