A study was made of the effect of lithium carbonate on the blood leukocyte count, absolute neutrophil and monocyte counts in 32 patients with neutropenia caused by the use of a specific treatment for lymphogranulomatosis (LGM), malignant lymphomas and multiple myeloma (MM), and in 5 patients with hypoplastic anemia (HA). Administration of lithium carbonate led to a significant increase in the total leukocyte and neutrophil counts in patients with LGM, malignant lymphomas and MM but not in patients with HA. The rise of the neutrophil count detected before (after 3 days) suggests that lithium may exert a direct stimulant action on relatively mature myeloid cells (promyelocytes and even myelocytes) but not on the undifferentiated colony-forming cells. The exponential dependence was found between an increase in the neutrophil count and duration of lithium intake, which permits forecasting the expected rise in the counts of these cells.