A long-term suspension culture line (c-WRT-7) was successfully established from a transplantable myelomonocytic leukemia induced by a neonatal injection of Rauscher leukemia virus in a WKA/Hok rat. A c-WRT-7 cell line was capable of being transplanted into syngeneic rats, and when transplanted, increased numbers of macrophage-like cells were observed in the peripheral blood of rats after i.v. injection. In in vitro culture, about 10% of the c-WRT-7 cells naturally differentiated into macrophage-like cells, which adhered to the bottom of a culture flask, and also possessed phagocytic activity. By means of cytological examination, about 30% of the c-WRT-7 cells were observed to be monoblastic with alpha-naphthyl butyrate esterase activity. The nature of these c-WRT-7 cells as a myelomonocytic leukemia line was constant during in vitro passages of more than 30 generations. In vitro treatment of c-WRT-7 cells with lipopolysaccharide, 12-O-tetradecanoylphorbol-13-acetate, or retinoic acid increased the numbers of differentiated cells with phagocytic activity to 80%. Treatment of the c-WRT-7 cells with the inducers also induced 15 to 20% of the cells to differentiate into metamyelocytes and segmented neutrophils. The Fc receptor and the complement receptor both became detectable on the surface of c-WRT-7 cells after treatment with lipopolysaccharide, 12-O-tetradecanoylphorbol-13-acetate, or retinoic acid. However, rosette-forming activity of sheep erythrocytes pretreated with neuraminidase which has been known as a marker of normal rat macrophages was not induced in c-WRT-7 cells. This shows that differentiated leukemic cells are not exactly identical with normal macrophages.