Peptidoglycan monomer (dissacharide-pentapeptide, PGM) fromBrevibacterium divaricatumis an immunomodulator and an antitumor agent. As part of our investigation of the antitumor properties of PGM in mice, its potential to stimulate phagocytosis by resident peritoneal macrophages was assessed. Inbred CBA/H/Z Tmice (kept under conventional conditions) were used, and a simple and brief method was developed to evaluate phagocytosis. It consisted of a short (10 min) coincubation of yeast cells (YCs) with peritoneal cells (PCs) and microscopic (phase-contrast) scoring of YC ingestion. Three samples of PGM were injected intravenously into mice (60 mg/kg), and PCs were collected from groups of recipients 8, 16, 24, 48, or 72 h later. All samples temporarily increased phagocytosis, but the extent and duration of the effect varied. Stimulation of phagocytosis also occurred after in vitro exposure (3 h) of PCs to PGM, or to the pentapeptide (PP) part of its molecule. We concluded that PGM could enhance phagocytosis by resident peritoneal macrophages in vivo and in vitro, the PP being the active component in vitro.