Ten patients with active acute myelogenous leukemia (AML) received either 13 cis retinoic acid (RA) + alpha interferon (IFN) or recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) for 3 days. Cell cycle measurements were performed before and at the conclusion of administration of the bioactive agent(s). The proliferative rate of the leukemia cells in vivo decreased in four of five patients receiving RA+IFN whereas in one patient proliferation accelerated. The proliferative rate of AML cells accelerated in three of the five patients who received rhGM-CSF and slowed in two patients. These data show that while the proliferative rate of AML cells can be altered in vivo, the effect produced by bioactive agents may be the opposite of the desired effect. Furthermore, the studies described here demonstrate the usefulness of marrow biopsies for measuring the percent S-phase cells and the importance of measuring the duration of S phase so that the effects of bioactive agents on the cell cycle time of the leukemia cells can be determined.