Soluble effector molecules involved in the rejection of allografted mouse Meth A (3-methylcholanthrene-induced ascites type tumor) cells were examined. A potent antiproliferative activity against the tumor cells was detected in the culture supernatant of leukocytes that had infiltrated into the peritoneal cavity of C57BL/6 mice on days 6-8 after transplantation, when the i.p. transplanted Meth A cells were undergoing rejection. Studies with neutralizing antibodies indicated that both IFN-gamma and IL-1 alpha/beta were required for the activity but that TNF-alpha was not. Actually, mouse rIFN-gamma and rIL-1 alpha/beta used, at the concentrations found in the culture supernatant determined by ELISA, were able to reconstitute the potent antiproliferative activity, although each cytokine alone had a weak growth inhibitory activity against Meth A cells. The half-maximal inhibition was observed with 0.02-0.03 U/ml of rIL-1 alpha/beta at 1-100 U/ml of rIFN-gamma. The synergistic growth inhibitory effect of these cytokines also was observed with four of 15 mouse transformed cell lines tested, indicating that the effect was not specific to Meth A cells. These findings suggest that IFN-gamma and IL-1 alpha/beta participate as soluble effector molecules in the rejection of some allografted tumor cells including Meth A cells.