By using linkage to transcriptional enhancers to increase gene expression, it was shown that the introduction of the mutant Ha-ras gene from bladder carcinoma cells into primary cultures of Chinese and Syrian hamster cells, and Wistar rat cells, was sufficient to trigger malignant conversion. The normal Ha-ras gene when linked to enhancers did not trigger malignant conversion but gave rise to cell clones with extended growth capacity. We measured the levels of transforming growth factors (TGFs) released from cells containing the ras genes, and the availability of the receptor for epidermal growth factor (EGF) to which alpha-TGF also binds. The level of ras gene expression seemed to correlate with the level of TGF secreted, a reduction in available EGF receptors and the morphology of the transformed cells. However, when an immortalized cell line (rat 208F cells) was used as a recipient for the oncogene vectors a different result was obtained. TGF release was triggered to varying degrees by almost all of the ras and myc oncogene constructions tested, regardless of the level of gene expression. Enhanced expression of the two oncogenes led to increased anchorage-independence. Significantly, both normal ras and normal myc, when linked to transcriptional enhancers, caused the recipient cells to become tumorigeneic without inducing any marked morphological alterations in culture. Enhanced and constitutive expression of the mutant Ha-ras gene resulted in the recipient cells becoming tumorigeneic and the cells displayed a transformed phenotype in culture. The transfected fibroblasts with a markedly transformed phenotype displayed a relative paucity of EGF receptors which correlated with release of alpha-TGF, suggesting a down-regulation of EGF receptors on transformation. While enhanced expression of the normal Ha-ras gene did result in a decrease in EGF receptors on these transfectants, in contrast, the transfectants with enhanced expression of the c-myc and v-myc oncogenes did not display any significant changes in available EGF receptors although the cells were tumorigenic when tested in the nude mouse. These results are discussed in relation to a hypothesis implicating a role for oncogenes at various stages in the multi-stage process of carcinogenesis.