The antileukemic activity of murine recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) and a combination of rGM-CSF and recombinant interleukin-3 (rIL-3) was examined by using a murine model of spontaneous B-cell leukemia (BCL1) in BALB/c mice. All untreated mice inoculated with 2 x 10(2) BCL1 cells developed leukemia within 4 weeks, with extreme lymphocytosis and a massive increase in both spleen weight and cell number while the number of myeloid progenitors (CFU-C) per spleen was decreased. In contrast, rGM-CSF-or rGM-CSF- and rIL-3-treated recipients did not show any evidence of leukemia or splenomegaly at 4 weeks and showed a significant increase in CFU-C per spleen. Hematologic parameters in the peripheral blood of untreated mice showed anemia and thrombocytopenia. Significant elevations in these parameters were recorded in mice treated with either protocol of CSF. Treatment of recipient mice with either rGM-CSF or rGM-CSF and rIL-3 prolonged their median survival from 6 weeks in untreated controls (range, 5 to 9 weeks) up to the time they were killed at 105 days. Adoptive transfer of spleen cells obtained from mice treated with rGM-CSF, mice treated with a combination of rGM-CSF and rIL-3, and untreated controls, into normal secondary recipients indicated improved survival in recipients inoculated with rGM-CSF. These data indicate that CSFs may inhibit in vivo expansion of leukemic cells of lymphoid origin.