In order to study the pathogenesis of juvenile-type chronic myelocytic leukemia (CML), we examined the colony-forming capacity and colony composition in the bone marrow (BM) and peripheral blood (PB) of three children with juvenile-type CML. Large numbers of granulocytes and macrophage colonies were formed by BM and PB cells. Whole agar culture staining revealed that especially macrophage colonies increased in comparison with normal controls. After removal of carbonyl iron-laden cells with a magnet or deprivation of cells adherent to glass from BM cells, the number of macrophage colonies markedly reduced in comparison with the number of colonies formed by untreated BM cells, suggesting that some of the macrophage colony-forming cells (M-CFC) may have phagocytic and/or adherent activity. Radiation sensitivity and thymidine suicide rate of these M-CFC were not different from those of granulocyte colony-forming cells (G-CFC). The predominance of M-CFC in juvenile-type CML may be one of the reflections of fetal-type myelopoiesis since M-CFC are predominant in cord blood and PB in the neonatal period. Moreover, considerable numbers of erythroid-colony-forming units (CFU-E) were present in PB of all patients. It may be concluded that juvenile-type CML is a panmyelopathy with the predominance of M-CFC.