Kinetic studies of leukemic blast cells (LBC) and erythroblasts were carried out in 20 cases of acute myeloblastic leukemia (AML), and the results were compared with those observed in dividing granulocytic precursors (DGP) and erythroblasts in a control group of 16 normal subjects. The parameters studied were: mitotic index (MI), labelling index (LI-3HTDR), MI/LI ratio and mitotic time (MT). The results showed that (1) LBC of AML do not proliferate more quickly than normal DGP, on the contrary, they replicate more slowly; (2) LBC obey some 'ecological laws' of normal hemopoiesis as a response to the control of the proliferative activity and release through the 'marrow-blood barrier'; (3) the difference between hyperleukocytemic and subleukemic AML is not related to the multiplicative activity of marrow LBC, but rather to other unknown biological factors; and (4) the kinetic behavior of erythroblastic population in AML is consistent, at least in some cases, with its 'leukemic origin'.