A transforming growth factor was found in the extracts of leukemic cells obtained from the peripheral blood of 11 patients with leukemia. This factor stimulated the colony formation of anchorage-dependent BALB/c 3T3 cells in soft agar. The high levels of colony-stimulating activities were observed in the cell extracts from patients with chronic myelogenous leukemia in blastic crisis (CML BC), acute myelogenous leukemia and CML in chronic phase. The factor from a CML BC patient was heat- and acid-labile, relatively stable to dithiothreitol treatment and inactivated by pronase treatment. Molecular size of the factor seems more than 10,000 daltons.