Summary It is well known that MPSV induces myeloproliferative syndrome (MPS) in mice. Intravenous one shot inoculation of myeloproliferative sarcoma virus (MPSV) with Friend murine leukemia virus (F-MuLV) as a helper in newborn Jar-2 rats (on the second neonatal day) yielded hematopoietic malignancies in all the treated rats (25/25 rats) after 2 weeks' latency. MPS appeared from the 14th day in 14 rats. In the midst of the myeloproliferative field of the spleen and bone marrow, myeloblastic or myeloblastic-erythroblastic foci were observed. From 19th day, acute myeloblastic leukemia occurred in 3 rats and erythroleukemia in 8 rats. MPSV induced first MPS which remained as such or later developed into acute leukemia. Myelofibrosis as seen in mice was not observed. In addition, hemangiosarcoma of the brain, spinal cord and spleen appeared in 15 rats from the 24th day, and were often multiple. MPSV can yield the tumor only in newborn rats, and target cells of MPSV are not only hematopoietic cells but also endothelial cells of the brain, spinal cord and occasionally spleen.