The administration of lithium has been associated with granulocytosis in psychiatric patients and those with Felty's syndrome. Lithium can attenuate neutropenia in patients receiving cancer chemotherapy (1). The clinical significance of these observations depends on whether lithium increases production of granulocytes or merely delivers to the circulation granulocytes that are marginated or are in the marrow reserve. In this study we measured the circulating and marginated granulocytes and the marrow granulocyte reserves in volunteers before and after the administration of lithium to determine the mechanism of lithium-induced granulocytosis. Six healthy volunteers, aged 24 to 31 years, received lithium carbonate, 300