Treatment of newborn male C3H/HeJ mice with N,N-diethyl-nitrosamine (DEN) or N-ethyl-N-nitrosourea (ENU) resulted in the induction of hepatocellular adenomas and carcinomas with a mean number of tumors per animal that was approximately 20- to 50-fold higher than that for similarly treated C57BL/6J male mice. We used two methods to study the genetic basis for this difference in susceptibility to liver tumor induction. Analysis of DEN-induced liver tumor multiplicities as a quantitative genetic trait in segregating crosses between C3H/HeJ and C57BL/6J mice indicated that allelic differences for at least two loci contributed to the higher sensitivity to hepatocarcinogenesis of C3H/HeJ mice relative to C57BL/6J mice. However, a single locus, which we have denoted Hcs (hepatocarcinogen sensitivity), was responsible for approximately 85% of the difference in susceptibility. The C57BL/6J and C3H/HeJ alleles at this locus were semi-dominant. This result was confirmed by analysis of hepatocarcinogenesis by ENU in BXH (C57BL/6J X C3H/HeJ) recombinant inbred mice. Four of the nine recombinant inbred strains studied were highly susceptible to the induction of liver tumors by ENU, three of the strains exhibited the resistant phenotype of the C57BL/6J parent, and two of the strains were of intermediate sensitivity to hepatocarcinogenesis. Newborn male C3H/HeJ and C57BL/6J mice did not significantly differ in the extent of ethylation of hepatic DNA, or in the relative levels of N-7-ethylguanine or O6-ethylguanine after treatment with [1-14C]DEN.