We have developed a system for expressing bcr/abl genes in the mouse hematopoietic system utilizing retroviral gene transfer and bone marrow transplantation. Expression of the P210bcr/abl gene in mice gives rise to a spectrum of hematological malignancies, most prominently a myeloproliferative syndrome which closely resembles human chronic myelogenous leukemia (CML). Studies of this system and related systems in other laboratories have begun to yield insights into the pathophysiology of the human bcr/abl leukemias. The CML-like syndrome appears to be a consequence of infection of a multipotential hematopoietic progenitor target cell. The leukemic clone is difficult to transplant to secondary recipients, but undergoes evolution to acute leukemia. The P190 form of bcr/abl appears to be more potent in leukemogenesis than P210, but may also be associated with a CML-like picture upon infection of a multipotential target cell. There may be a spectrum of different chronic phase duration associated with different Bcr/Abl proteins, with bcr sequences influencing the rate of disease progression. In mice, duplication or alterations of the bcr/abl gene itself may constitute a major mechanism of disease progression.