The administration of recombinant human macrophage colony-stimulating factor (M-CSF) i.p. in doses of 25 or 100 micrograms twice daily for 5 consecutive days to non-tumor-bearing C57BL/6 mice resulted in a dose-dependent infiltration of mononuclear cells in the livers but not the lungs of these treated animals. Immunohistochemical examination of fixed liver tissue with the murine macrophage-specific monoclonal antibody, F4/80, revealed a greater than 5-fold increase in the number of hepatic macrophages. Quantification of F4/80-positive cells in a mononuclear single cell suspension derived from liver revealed a greater than 25-fold expansion in the number of hepatic macrophages compared to control mice. These cells were then tested in 18-h 51Cr release assays for tumoricidal activity, after an 18-h incubation with or without gamma-interferon, against cultured P815 targets. Significant tumor cell lysis by these liver-associated mononuclear cells occurred, which was enhanced by gamma-interferon preincubation. The systemic administration of M-CSF alone at high dose had no antitumor impact in vivo against 3-day pulmonary metastases from the MCA-203 sarcoma and B16 melanoma or hepatic metastases from the B16 melanoma or MCA-105, -203, or -207 sarcomas. Although the systemic administration of M-CSF in combination with tumor-specific monoclonal antibody had no effect on 3-day pulmonary metastases from the B16 melanoma, significant reductions in liver metastases were seen. These murine studies demonstrate the biological activity of recombinant human M-CSF in vivo and suggest that the administration of this cytokine in combination with specific monoclonal antibody may be useful in the treatment of patients with metastatic disease at sites of monocyte/macrophage accumulation.