The mechanism of rejection of allogeneic organ grafts is not fully understood. The rejection of grafted skin is complicated as skin epidermal cells express skin specific antigens. Thus skin epidermal cells are suitable target cells for analyzing skin graft rejection in in vitro studies. In this study, (51)Cr labeled skin epidermal cells were prepared from rat tail skin by trypsin digestion. The susceptibility of skin epidermal cells to immune spleen cells in a lymphocyte mediated cytotoxicity assay and LDA in an antibody dependent cytotoxicity assay was examined in allogeneic skin grafts in rats. Skin epidermal cells were specifically killed by immune killer lymphocytes or LDA in combination with normal rat spleen cells. The optimal condition for assays were 10 to 12 hours of incubation and a 100:1 to 200:1 effector to target cell ratio. When the kinetics of LDA activity after allogeneic skin graft was examined, LDA appeared in the recipient serum just after the rejection and maintained activity for more than 40 days after the skin graft. To analyze the mechanism of graft rejection, it is resonable to use the same target cells as the graft itself in both cellular and humoral immunoassays. In this sense, skin epidermal cells are useful for the study of skin rejection.