We have assessed the role of epidermal growth factor receptor (EGFR) signaling in biological responses to the v-rasHa oncogene using primary keratinocytes from Egfr -/- mice and wild-type littermates. On the basis of several criteria, Egfr -/- keratinocytes were unresponsive to either acute or chronic exposure to several EGFR ligands but were stimulated to proliferate in response to several other mitogens. Although conditioned medium from primary keratinocytes transduced with v-rasHa retrovirus (v-rasHa keratinocytes) was a potent mitogen for wild-type but not Egfr -/- keratinocytes, v-rasHa transduction of primary keratinocytes of either genotype resulted in a strong mitogenic response, arguing against an obligatory role for EGFR activation in v-rasHa-mediated stimulation of keratinocyte proliferation. Infection with high-titer v-rasHa retrovirus altered the keratin expression pattern in keratinocytes of both genotypes, suppressing differentiation-specific keratins K1 and K10 while activating aberrant expression of K8 and K18. In wild-type but not Egfr -/- cultures, K1 and K10 were also suppressed following infection at lower retroviral titers, presumably as a result of paracrine EGFR activation on uninfected cells present in these cultures. Squamous papillomas produced by grafting Egfr -/- v-rasHa keratinocytes onto nude mice were only 21% of the size of wild-type v-rasHa tumors, and a striking redistribution of S-phase cells was detected by immunostaining for bromodeoxyuridine. In Egfr -/- v-rasHa papillomas, the fraction of total labeled nuclei detected in suprabasal layers was increased from 19 to 39%. In contrast, the basal layer labeling index of Egfr -/- papillomas was reduced to 34%, compared to 43% in wild-type tumors. Our results indicate that, although autocrine EGFR signaling is not required for keratinocyte responses to oncogenic ras in culture or benign tumor formation in nude mouse grafts, disruption of this pathway impairs growth of v-rasHa papillomas by a mechanism that may involve alterations in keratinocyte cell cycle progression and/or migration in vivo.