The ability of several species of black-pigmented Bacteroides to degrade immunoglobulins A and G was confirmed in this study. The cleavage products from IgG strongly stimulated the growth of bacteria degrading IgG. Growth of Bacteroides gingivalis on limiting media supplemented with IgG paralleled growth on complete medium. The degradation of IgG and IgA by black-pigmented Bacteroides appeared to occur in 2 stages. The molecules were broken into large fragments which were subsequently degraded into small peptides not visible on SDS-PAGE. B. gingivalis degraded IgG to peptides with Mrs of 33,000 and 11,000 whereas Bacteroides asaccharolyticus, Bacteroides intermedius and Bacteroides loescheii formed only the 33,000 Mr peptide. Electrophoresis in polyacrylamide gels containing covalently linked IgG, IgA and bovine serum albumin revealed that B. gingivalis elaborated 8 electrophoretically distinct proteolytic activities. The proteases protected the cell from reaction with anti-B. gingivalis antibody and were capable of hydrolyzing antibody bound to the bacterial cell surface.