Granulocyte kinetic studies, using radioactive diisopropylfluorophosphate (DF32P), were performed in 15 patients with acute myelogenous leukemia (AML) and 10 patients with the myelomonocytic variety (AMML). Attention was focused upon the more differentiated cells present in the blood of the leukemic patients. The patterns of disappearance of in vitro labeled leukocytes from the blood, and the patterns of emergence of in vivo labeled leukocytes from the bone marrow were variable. Nonuniformity in the results suggests that the terms AML and AMML cover a spectrum of disorders, and that broad generalizations concerning their kinetics cannot be made. The implications of these findings are discussed, and new evidence suggesting that the mature neutrophils in the blood of leukemic patients may arise from abnormal or leukemic precursors is presented.