The expression of proliferation-associated genes, cdc2 and E2F-1 and squamous-specific genes transglutaminase type I and cornifin were examined in senescing human epidermal keratinocytes. Cultured keratinocytes underwent 34 population doublings before senescing. Senescence in keratinocytes was characterised by reduced thymidine incorporation, a change in morphology and the inability of cells to undergo mitosis. This was accompanied by downregulation of cdc2 and E2F-1 mRNA's. In addition, senescing keratinocytes started to express genes such as cornifin, that are specific for squamous differentiation. These changes were similar to those observed in keratinocytes induced to differentiate with phorbol ester or by confluence. E2F-1, cdc2 and cornifin were similarly altered in senescing human mammary epithelial cells. Our data suggest that events regulating senescence may also be linked to squamous differentiation. © 1993 Academic Press, Inc.