Mice inoculated with the RFV strain of Friend virus develop an erythroleukemia that spontaneously regresses. Bone marrow cells from leukemic mice produce a factor that inhibits colony formation in agar by normal CFU-Cs. CFU-Cs from leukemic animals are resistant to the factor. Bone marrow cells from some mice whose leukemia has regressed continue to produce the factor, and CFU-Cs from some regressed mice retain resistance to the factor. These observations dupllicate findings reported for patients with acute leukemias and those with leukemia in remission. The RFV system thus serves as an accurate experimental model for the study of hematopoietic cell interactions and the proliferative advantage of neoplastic cells in human leukemias.