Peripheral leukocytes from a male patient with chronic myelogenous leukemia in blast crisis were implanted directly into the intraperitoneal cavity of seven newborn hamsters treated with antilymphocyte serum. At sacrifice 17–19 days after implantation, three hamsters were found to have disseminated tumors. The other four hamsters were cannibalized. Chromosome analysis of cells from the enlarged lymph nodes revealed the presence of a human male karyotype. Most metaphase cells were pseudodiploid with the Ph1 chromosome and isochromosome 17, while a few metaphase cells were classically diploid. Identical chromosome constitutions were observed in the original peripheral leukocytes of the patient. The results strongly suggested simultaneous heterotransplantation of Ph1-positive leukemic and normal leukocytes.