Seventy Bacteroides strains isolated from clinical materials were tested in vitro against nine antibiotics by means of modified disc diffusion and agar dilution tests. Correlating the agar dilution susceptibility with the concentrations achievable in serum for each drug tested, we found that at least 90% of the strains were susceptible to clindamycin, chloramphenicol, carbenicillin, and lincomycin. Only 40% were susceptible to tetracycline. Except for tetracycline and chloramphenicol, the disc diffusion technique exhibited inadequate predictive value in terms of “susceptibility” and “resistance” for the Bacteroides strains tested.