Keratin intermediate filaments are heteropolymers of coexpressed type I and type II protein chains, whose expression is tightly linked to the differentiation status of the keratinocyte. Epidermal basal keratinocytes coexpress keratins K5 and K14, whereas suprabasal keratinocytes downregulate K5 and K14 and begin to coexpress keratins K1 and K10. Using both isotopic and non-isotopic in situ hybridization, we have investigated the changes in expression of the messenger RNA species encoding the K5/K14 and K1/K10 keratin pairs in response to ultraviolet radiation. Here we report that following irradiation, the mRNA species encoding both keratin pairs is upregulated in a wavelength-specific manner, and that the link between the pattern of keratin mRNA expression and the differentiation status of the keratinocyte is disrupted. Forty-eight hours following ultraviolet B exposure, the amount of detectable mRNA encoding all four keratins studied had increased. Following UVA irradiation, the K1 and K10 signal increased to a much lesser extent than following ultraviolet B, whereas no change in the amount of mRNA encoding the K5/K14 pair was observed. Only two samples were examined following ultraviolet C exposure, but in both, increased K5/K14 signal, but not suprabasal K1/K10 signal, was observed. We suggest that the observations reported here may reflect important qualitative changes involved in photoadaptation of the epidermis, and provide further molecular markers of the different biological effects of ultraviolet radiation of different wavelengths.