Culture of bone marrow and/or blood cells in a semisolid agar system from 43 adults with acute nonlymphoblastic leukemia at first presentation showed two distinct growth patterns at 14 days. In 53% of patients cells failed to grow (type O), while in the remainder an abnormal growth pattern (type B) with small numbers of diffuse colonies and excessive numbers of cell clusters was seen. The response following chemotherapy was significantly better in the patients whose cells failed to grow. Serial culture studies, performed in 9 patients throughout remissions of 100–1112 days, which had been maintained by intermittent chemotherapy, showed wide fluctuations in proliferative activity. These ranged from no growth to marked proliferation with predominance of clusters and small numbers of diffuse colonies, indistinguishable from the type B pattern seen in 47% of patients at first presentation. The possibility is discussed that the periods of failure to grow, and/or those in which a type B pattern emerged, represented sporadic reactivation of leukemic cells.