Gamma interferon (gamma-IFN), a lymphokine produced by activated T lymphocytes, has a variety of effects on target cell. It induces class II antigens of the major histocompatibility complex not only in immunocompetent cells but also in non-immunocompetent cells. gamma-IFN also can exert, in addition to anti-viral activity, a series of anticellular effects on a variety of cell types. The effects of gamma-IFN on the proliferation of cultured epidermal cell (EC) and induction of HLA-DR antigen expression by EC (HLA-DR+KC) were studied. Furthermore, the immunologic role of HLA-DR+KC in the mixed epidermal cell-lymphocyte reaction (MECLR) was studied. The antiproliferative effect of gamma-IFN on the cultured EC was seen 3 days after treatment of gamma-IFN and the effect was dose-dependent. Number of HLA-DR+KC was increased dose-dependently with treatment of gamma-IFN. In MECLR, HLA-DR+KC had been found to exert stimulatory role on allogenic lymphocytes. However, there was no significant role of HLA-DR+KC on autologous lymphocytes.