Using an avidin:biotin immunoperoxidase system with monoclonal antibodies to lymphocyte subsets, we have investigated the host response to malignant melanoma and melanocytic nevi in frozen sections. Eight primary melanomas, eight metastases, three dysplastic nevi, and two dermal nevi were studied with antibodies T11, T4, T8, and B1. Sections were read in a semiquantitative manner by two observers. Virtually all lymphocytes in these lesions were T-cells (T11 positive). In all primary melanomas, in the majority of metastases, and in all dysplastic nevi, both T4- and T8-positive cells were present. In two of eight metastases, tumor cells stained with T4, and in one case, melanoma cells stained with B1 antibody. The host response to melanoma involves primarily T-cells and includes both the helper:inducer (T4) and suppressor:cytotoxic (T8) subsets.