Immunohistologically, cellular infiltrates in regressing plane warts were mainly composed of lymphocytes and mononuclear phagocytes. There were many infiltrating T lymphocytes. Immunoelectron microscopic observation demonstrated that both helper/inducer and suppressor/cytotoxic phenotypes of T lymphocytes infiltrated in the lesions. OKT6-positive cells were observed in the dermis as well as in the epidermis. Moreover, as noted in allergic contact dermatitis, the apposition of T lymphocytes to Langerhans' cell-like cells could be seen. Lymphocytes and a small number of mononuclear phagocytes were found adjacent to damaged keratinocytes in the epidermis, the picture of which has been described as satellite cell necrosis, a hallmark of cytotoxic reaction by aggressors. These findings suggest that specific cell-mediated immunity against virus-infected keratinocytes takes place in the process of regressing plane warts.