The characteristics of proliferation of leukemic cells in four children with untreated acute leukemia have been studied. In all four of these children a population of marrow leukemic cells was found which were dividing with a generation time of about 15 to 20 hours. In two of these patients it was possible to demonstrate that these dividing cells after one or more mitotic divisions became smaller and stopped dividing. In all of these patients 70 per cent or more of the leukemic cells of the marrow and almost all leukemic cells of the blood were nonproliferative at the time of these studies. These nondividing cells would be relatively unaffected by chemotherapeutic agents designed to inhibit cell division.