The controversy surrounding the interpretation of observed increases in the high spontaneous liver tumor incidence of the B6C3F1 mouse after administration of certain chemical agents necessitates a mechanistic understanding into the nature of tumor development in this particular strain of mouse. Recently, cancer genes (oncogenes) have been detected in the DNA from a variety of human tumors and tumor cell lines. These genes have been implicated to play a role in the transformation of normal cells into cancerous ones. To investigate the role that cellular oncogenes might play in the development of spontaneous liver tumors in the B6C3F1 mouse, DNA was isolated from spontaneously occurring liver tumors and transfected into NIH 3T3 fibroblasts. DNA from this tumor tissue was capable of transforming NIH 3T3 cells from 82% of the animals examined strongly suggesting the presence of an active cellular oncogene. In contrast, DNA isolated from surrounding non-tumorous liver tissue and liver tissue from non-tumor bearing mice did not cause any transformation in the NIH 3T3 assay. These data demonstrate that the active cellular oncogene is not present in the hepatic tissue via a germ-line transmission but is activated only in those cells of the tumor tissue. Experiments using Southern blot hybridization analysis have identified this active cellular oncogene to be a member of the ras oncogene family. Identification of this cellular oncogene will now allow the evaluation of factors which might modify its expression. These future studies will lead to an increased understanding of potential mechanisms by which hepatic tumors are enhanced and should provide more informed estimates of risk for man based on bioassay data generated in this strain of mouse.