Recent studies by several authors suggested that Bacteroides fragilis and related species are becoming more resistant to tetracyclines. Therefore, the minimal inhibitory concentrations of tetracycline and doxycycline were determined by broth dilution tests for a total of 80 Bacteroides strains (56 B. fragilis, 15 B. thetaiotaomicron, 9 B. vulgatus), 70 of which had been isolated from clinical specimens during 1973/74. Using initial concentrations of 1 x 10(8) organisms per mo of thioglycolate broth the MIC were determined after 5 hr of incubation. Thus the influence of thermal inactivation of tetracycline compounds on the results obtained was minimized. Tetracycline at less than 0.00075 through 12.5 mug/ml was active against 79 strains. The MIC of one strain, a member of B. fragilis, was 50 mug tetracycline/ml. At 1.56 mug doxycycline/ml the cumulative percentage of sensitive strains was 97,5. The MIC of only 2 Bacteroides cultures was 3.12 mug doxycycline/ml. Taking account of the relatively high initial concentrations of test organisms, the results obtained may be estimated at the quotient of 1/4 as representative of the usual inoculum of 10(6) organisms/ml. Accordingly, 79 and 80 strains, respectively, may be considered to be sensitive to tetracycline and doxycycline in clinically achievable levels.