Oral lithium carbonate therapy produces a reversible neutrophilic leukocytosis (Murphy et al., 1971; Shopsin et al., 1971). What was initially regarded as a medical curiosity in psychiatric patients has been exploited therapeutically in the treatment of neutropenia (Gupta et al., 1976; Schapria et al., 1977; Jacob and Herbert, 1974; Barrett et al., 1977a) and chemotherapy induced bone marrow suppression (Turner and MacDonald, 1979). A true increase in granulocyte mass occurs with lithium ingestion rather than a shift of neutrophils from the marrow reserve or demargina-tion of granulocytes (Rothstein et al., 1978; Stein et al., 1978). Elevated urinary and serum colony stimulating activity (CSA) has been reported in neutropenic patients on lithium (Gupta et al., 1976) and lithium has been shown to augment CSA production in vitro from mouse lung (Harker et al., 1977) and mononuclear cells (Joyce and Chervenick, 1975). A major source of CSA in man are the peripheral blood mononuclear cells (Chervenick and LoBuglio, 1972). These studies examined the effects of oral lithium carbonate on human mononuclear cell production of CSA as measured by its capacity to stimulate human granulocyte-macrophage colony formation.