In order to study the changes in erythroid precursor cells in erythroleukemia, bone marrow cells from 4 patients were cultured for erythroid colony-forming units (CFU-e). The bone marrow from 3 of the patients showed an excess of ringed sideroblasts, and the incidence of CFU-e was very low. The one patient without ringed sideroblasts showed abundant erythroid colony formation. Erythroid colonies of more than eight erythroblasts consisted of normal-appearing erythroblast, while ringed sideroblasts were observed in scattered erythroblasts or in small clusters. The number of granulocyte macrophage colony-forming units (CFU-GM) was markedly lower than normal in all 4 cases. In 2 cases investigated, blast colonies were formed from bone marrow cells under the stimulation of phytohemagglutinin/leukocyte-conditioned medium. These results show that the differentiation of hemopoietic stem cells to erythroid as well as to myeloid cells is affected in erythroleukemia.