Macrophage colony-stimulating factor (M-CSF) has been previously shown to facilitate the in vitro survival and differentiation of mononuclear phagocytes. We assessed whether M-CSF administration in vivo could induce macrophages capable of killing tumor via an antibody-dependent mechanism. C57BL/6 mice were given intraperitoneal M-CSF, and peritoneal macrophages were assayed for their ability to kill fluorochrome-labeled R1.1 thymoma cells in vitro in the presence or absence of target-specific antibody. Two-color flow cytometry was used in measuring tumor ingestion by macrophages; macrophages from M-CSF-treated mice eliminated greater than 90% of R1.1 thymoma target within 24 hours, while macrophages from saline-treated controls were ineffective. R1.1 tumor elimination by macrophages depended on the presence of target-specific antibody. These are the first studies that demonstrate the in vivo induction, by M-CSF, of macrophages directly capable of ingesting antibody-conjugated tumor cells.