The literature dealing with the pathogenicity of anaerobic gram-negative rods in humans is reviewed. Knowledge concerning definite pathogenic mechanisms is, at best, cursory. There is evidence that encapsulation plays a role in the pathogenicity of Bacteroides fragilis and some of the black-pigmented Bacteroides. A range of enzymes, among them collagenase and IgA protease, are produced by several Bacteroides species. Supernatants of Fusobacterium necrophorum cultures may be leukotoxic. Synergism between anaerobic gram-negative rods and other bacterial species has been demonstrated in experimental animals.