The colony and cluster forming capacity of bone marrow cells (BM CFC and CluFC) in agar culture, was studied from 20 oligoblastic patients. 13 patients had a leukemic growth pattern and 12 of these died within one year after diagnosis. 7 patients had no leukemic growth and 4 are alive 16 to 90 months after diagnosis. Both the determination of the proportion of abnormally light buoyant density of CFC and CluFC and the study of their suiciding index were used to characterize more precisely the leukemic or nonleukemic status of patients. Because of the small number of patients involved in the later preliminary study, the prognostic significance cannot be valuated. Serial studies of individual patients showed different types of evolution in the growth pattern of BM CFC and CluFC. Either the increase of BM CFC and CluFC paralleled that of the myeloblasts, or there was a change in the growth pattern before AML transformation suggesting clonal evolution.