The purpose of this study was to examine the phenotype of the cutaneous immunocompetent cells and Langerhans cells in malignant and benign tumour infiltrates (basal cell and squamous cell carcinomas, malignant melanoma, seborrheic keratosis and naevus) by the use of monoclonal antibodies directed against T cell populations and Langerhans cells. This in situ investigation indicates that the lymphocytes participating in the inflammatory reaction around skin tumours are mainly of the cytotoxic/suppressor class. It suggests, moreover, that interaction occurs in the skin between T lymphocytes and HLA-DR suppressing cells. The in situ study of the inflammatory immune response should be a useful complement to in vitro investigations in the exploration of immune reaction against tumours of the skin.