Two patients with acute Kala Azar were studied with DF32P (diisopropylfluorophosphate) and three patients with 51Cr (chromate) in an attempt to delineate the mechanism producing neutropenia in this disease. The granulocyte life span was found to be reduced in all the patients with exception of one who was studied during Glucantim treatment. The surface radioactivity counts showed that the reduced granulocyte life span was due to pooling and probable destruction of granulocytes in the spleen and to a lesser degree in the liver. Bone marrow neutrophil reserve, evaluated by the response to the intravenous hydrocortisone hemisuccinate, was found to be markedly reduced in all patients. An enlarged marginal granulocyte pool indicated also that the neutropenia may be due to altered intravascular granulocyte distribution.