It has previously been demonstrated that the epidermis is a rich source of proinflammatory cytokines and growth factors and that complex interactions between these factors may affect inflammatory responses in skin. To investigate whether IL-10 (cytokine synthesis inhibitory factor) is part of this complex process, RNA was extracted from normal epidermis at various times after application of various chemicals to murine skin and mRNA signals for IL-10 were sought using a quantitative reverse-transcriptase-polymerase chain reaction technique. IL-10 signal strength was normalized to that of beta-actin in each sample. IL-10 mRNA signals were occasionally identified in normal epidermis but were uniformly enhanced 4 h after hapten application, and were maximal after 12 h. Contact allergens induced IL-10 mRNA signals whereas vehicles and irritants did not. Depletion of Langerhans cells, Thy-1+ dendritic epidermal cells, and T lymphocytes demonstrated that keratinocytes were the main source of IL-10 mRNA. IL-10 signals were also detected in mRNA derived from PAM 212 (spontaneously transformed keratinocyte) cells. IL-10 protein could be detected by immunoprecipitation, with an IL-10 mAB, of supernatants obtained 16 h after cultured epidermal cells were coupled with hapten. This study demonstrates that murine keratinocytes are capable of producing IL-10 mRNA and protein, and that signal strength of IL-10 mRNA is enhanced by hapten application.