Human hemopoietic pluripotent progenitors form multilineage colonies when cultured in methylcellulose with medium conditioned by leukocytes in the presence of phytohemagglutinin (PHA-LCM) and erythropoietin (EPO). We have examined their frequency, culture requirements and proliferative activity in 20 peripheral blood and 29 bone marrow specimens from patients with CML in chronic phase. Multilineage colonies developed under regular culture conditions in approximately 50% of all samples. The frequency ranged from 1-36 per 2 X 10(5) mononuclear cells of density less than 1.077 gm/ml. The requirements for PHA-LCM and EPO varied for patients with CML when compared to normal individuals; i.e., cells from some patients gave rise to mixed colonies with substantial erythroid components in the absence of PHA-LCM or without addition of the usually required EPO concentrations. The proliferative activity of CFU-GEMM was assessed using a short-term exposure to tritiated thymidine (3HTdR) prior to plating. The plating efficiency in all bone marrow and peripheral blood samples was reduced to 40-70% of the unexposed controls. In contrast, the plating efficiency after exposure to 3HTdR in normal individuals usually ranged from 70-90% of controls for bone marrow samples and from 85-100% of controls for peripheral blood samples. Thus, an increased proliferative rate of pluripotent hemopoietic progenitors is a consistent feature of CML patients. In addition, at least in some patients, different requirements for erythropoietin or PHA-LCM were observed when compared to normal culture conditions.