The Fas antigen is a cell surface protein that can mediate apoptosis in many cell types. Although its physiological function is still unclear, recent evidence indicates that this surface molecule is involved in apoptosis in the immune system and the liver. The epidermis is an organ that undergoes terminal differentiation with the eventual death of keratinocytes, and it has been suggested that this is a specialized form of apoptosis. In the present study, we examined whether or not the Fas antigen is involved in keratinocyte apoptosis. Immunoreactivity for the Fas antigen was found throughout the epidermis in normal human skin sections and cultured normal human keratinocytes, and mRNA for the Fas antigen was found to be constitutively expressed in normal epidermis and cultured normal keratinocytes by RT-PCR analysis. To determine whether the Fas antigen in keratinocytes is functional, we used a cytotoxic monoclonal antibody (mAb) against the Fas antigen to induce apoptosis. This antibody did not induce apoptosis of cultured keratinocytes even though they expressed the Fas antigen. We then tested the ability of several cytokines (TGF beta, TNF alpha and IFN gamma) to induce Fas-mediated keratinocyte apoptosis. Only pretreatment with IFN gamma followed by the addition of the anti-Fas mAb induced apoptosis, as assessed by cell viability, morphological changes and ultrastructural characteristics, suggesting that constitutive expression of the Fas antigen is not sufficient to induce apoptosis in keratinocytes and that keratinocyte apoptosis via the Fas antigen-mediated mechanism may require the activation of keratinocytes by IFN gamma, which is thought to be produced by activated T cells.(ABSTRACT TRUNCATED AT 250 WORDS)