The possible combined effects of the initiator diethylnitrosamine (DEN)+neutrons on the induction of foci, adenomas and carcinomas in the livers of C57BL/Cnb mice were evaluated. Four groups of infant mice were treated as follows: DEN alone, neutrons alone, DEN followed by neutrons and neutrons followed by DEN. Ten mice in each group were killed at 10-week intervals over 70 weeks. The following parameters were measured: body weight, liver weight, number and size of superficial macroscopic liver lesions, and number and total surface area of the different types of microscopic liver lesions. The rate of appearance of foci increased significantly at different times when a dose of 0.125 Gy of neutrons was administered 7 days before or after a dose of 1.25 micrograms of DEN. No significant differences were observed in the total surface area of foci and/or adenomas and carcinomas when increasing doses of neutrons were given 7 days before or after the administration of 1.25 and 2.5 micrograms of DEN.