Epithelioid cells, giant cells and mononuclear cells in cryostat sections of lesional skin from 5 patients with chronic cutaneous sarcoidosis were characterized using an indirect immunofluorescence test with monoclonal antibodies. OKM1 (monocyte) antibody stained both epithelioid cells and giant cells as well as some cells in the periphery of the granulomas. OKM1 which reacts with most monocytes in peripheral blood, did not react with tissue macrophages in sections of normal skin, liver, spleen and kidney. The results indicate that the epithelioid and giant cells in the sarcoid granulomas are derived from blood monocytes. OKT3 (pan T cell) antibody stained the majority of cells surrounding the epithelioid cell islands. Examination of adjacent sections showed that more OKT4+ (helper/inducer) than OKT8+ (suppressor/cytotoxic) cells were present in the sarcoid lesions examined.