A novel human cultured cell line, P39/Tsugane, was established from leukemic cells in the peripheral blood of a 69-year-old male with overt leukemia following myelodysplastic syndrome (MDS). P39/Tsugane cells were characterized by blastic appearance, presence of NaF-sensitive alpha-naphthyl butylate esterase activity, Fc gamma-receptor, C3-receptor, capacity to phagocytize sensitized erythrocytes, and reactivity with monoclonal antibodies such as OKT4, My4, VIMD5, MCS-2 and My7. These data indicate that P39/Tsugane cells are of myelomonocytoid nature. P39/Tsugane had a hypodiploid chromosome constitution with a gain of a consistent marker, 6q+, the presence of less consistent markers 9q+ and rcp(14;16), and random and non-random losses of autosomes: in accordance with the reported cytogenetic profiles of MDS, a representative karyotype of the present cell line is 45,XY,+del(6)(q15),9q+, t(14;16)-(q24;q21),-16,-17. P39/Tsugane cells were transplantable intraperitoneally into nude mice, and produced abdominal tumors and hemorrhagic ascites. These results indicate that P39/Tsugane is the first cultured cell line of myelomonocytoid nature to be derived from overt leukemia following MDS. Therefore, P39/Tsugane cells should be useful for studies on the differentiation of leukemia cells, the pathogenesis of MDS and in vitro-in vivo experimental chemotherapy.