Using murine monoclonal antibodies against human HLA-DR antigen and against human T cells, we investigated the indirect immunofluorescence staining pattern of involved and uninvolved skin from patients with psoriasis. The staining pattern of involved psoriatic epidermis is different from the pattern seen in uninvolved skin from the same patient and consists of scattered, single HLA-DR positive cells alternating with groups of HLA-DR positive cells. These HLA-DR positive cell clusters can be seen at any level of the epidermis. In some patients, the dermis of involved skin shows prominent accumulations of T cells which are HLA-DR positive and thus represent activated T cells.