Highly purified blood lymphocytes from sarcoidosis patients were characterized by rosette technique for the presence of receptors for complement and by indirect immunofluorescence for the presence of membrane-bound immunoglobulin (B cells). T cells were estimated by spontaneous rosette formation with sheep red blood cells. The proportion of B cells was significantly higher and the proportion of T cells significantly lower than in the control group. Marked lymphocytopenia was found in the patient group. As a consequence the total number of circulating B cells was normal whereas the total number of circulating T cells was significantly depressed. Large atypical mononuclear cells were found in most patients. One-third of these cells had receptors for human complement.