SummaryUsing a panel of monoclonal antibodies directed against keratins (PKK2. CK8.12 and KL1). the effects of ultraviolet B (UVB) and psoralen plus ultraviolet A (PUVA) irradiation on keratin expression in guinea‐pig skin were examined immunohistoehemically. Following irradiation, whether by UVB or PUVA, rapid alterations in the distribution pattern of keratins were observed in the epidermis. The alterations included the induction of basal cell‐type keratins (PKK2and CK8.12 staining) in the suprabasal layers, with concomitant reduction ofthe suprabasal‐type keratins (KL1staining). These alterations in keratin expression were observed during the period when DNA synthesis appears to be accelerated by ultraviolet light exposure (5 h–5 days after LIVB, and 2–10 days after PUVA irradiation). Therefore, these changes are probably reflections ofa proliferative or regenerative state of keratinocytes. This explanation was supported by the result of an experiment involving tape stripping of the epidermal horny layers, which also accelerates DNA synthesis hy keratinocytes. Immunohistochemistry appears to be a useful and sensitive method of detecting the effect of ultraviolet light on kerat