Immunocompetent cells in exacerbating untreated psoriasis vulgaris skin lesions were immunophenotypically studied by the application of a selection of monoclonal antibodies in a two-stage immunoperoxidase technique. Epidermal changes include: focal accumulation of immunoglobulins in the stratum corneum, as demonstrated by a mixture of monoclonal anti-kappa and anti-lambda antibodies; focal accumulation of OKM-1 positive but Mo-2 negative cells high in the epidermis, reflecting granulocytes in Munro's abscesses; a marked decrease in epidermal Langerhans cells with focal abnormal clumping and smaller dendrites, as demonstrated by monoclonal anti-HLA-DR and anti-T6 (OKT-6) antibodies; and, sporadic exocytosis of mainly T1 (Leu-1), T8 (Leu-2a) positive suppressor/cytotoxic T lymphocytes. The dermal infiltrates were found to consist mainly of partically activated T1 (Leu-1), T4 (Leu-3a) positive T-helper/inducer cells with a smaller compartment of T1 (Leu-1), T8 (Leu-2a) positive suppressor/cytotoxic lymphocytes. These cells were found in close apposition to T6 (OKT-6), HLA-DR positive Langerhans cells and further accompanied by a minor compartment of OKM-1, Mo-2 positive monocytes. No B-cells or plasma cells could be demonstrated in the dermis. Natural killer cells were observed only incidentally. These results fit best with the hypothesis that psoriasis is a chronic inflammatory condition as a result of persistent stimulation of T cells by immunogen(s) of epidermal origin.