The interactions of anaerobic bacteria with polymorphonuclear leukocytes (PMNs) are reviewed. Anaerobes, including species of Propionibacterium, Fusobacterium, and Bacteroides, produce serum-independentchemotactic factors capable of attracting PMNs. In addition, these bacteria produce chemoattractants by complement activation and possiblyby interaction with host lipids. Bacteroides species are optimally phagocytosed by PMNs whenopsonizedby both immunoglobulinand complementalthough the latter appears to provide the critical opsonin. As is the case with severalaerobic bacteria, the polysaccharide capsule of Bacteroides fragilis interferes with PMN phagocytosis and antibody to the capsule partially overcomes this virulence property. The presence of Bacteroides species may also inhibit PMN phagocytosis and killing of aerobic gramnegativebacteria either by competing for limitedopsonins or by decreasing intracellular killing. Bactericidal activity of PMNs against anaerobes appears to proceed primarily through oxygen-independent mechanisms, although the relevant microbicidal components have not been identified. Some antibiotics with activity against anaerobic bacteria penetrate PMNs, and antibiotic pretreatment of B.fragilis, with levels below the minimal inhibitory concentration, facilitates PMN phagocytosis of these organisms.