Cytochemical and immunological studies were performed on tissues and mononuclear cell suspensions from ten patients with hairy cell leukemia. In all cases studied, tartrate-resistant acid phosphatase was noted within the cytoplasm of hairy cells (HCs). In two thirds of the cases, alpha naphthyl acetate esterase was observed in HCs. In addition, HCs did not form spontaneous rosettes with sheep erythrocytes. A variable number of HCs displayed complement receptors. The nonspecific binding of conjugated immunoglobulin to HCs probably reflected the presence of a high concentration of Fc receptors on the HCs surface. In three cases, the conjugated immunoglobulin reacted predominantly to one light chain, thus suggesting the presence of a monoclonal immunoglobulin. In four of six cases, mononuclear cell suspensions of HCs demonstrated latex phagocytosis. In one case, HCs displayed resynthesis of surface membrane immunoglobulin, the presence of B cell antigen, and phagocytosis of latex. These findings suggest that HCs are distinctive and contain properties of both B lymphocytes and monocytes.