The minimal inhibitory concentrations of 601 clinical isolates of anaerobic bacteria to 10 different antimicrobial agents were determined by an agar-dilution technique. Nearly all strains were resistant to kanamycin and gentamicin, although moderate activity to both drugs was noted with Fusobacterium sp., anaerobic cocci, some strains of Bacteroides melaninogenicus, and nonsporeforming gram-positive bacilli. Chloramphenicol at 12.5 mug/ml inhibited all but three of the strains tested. Tetracycline at 6.25 mug/ml had high activity against all groups tested, with the exception that only 39% of strains of Bacteroides fragilis were inhibited at this concentration. Excluding certain species of Bacteroides, the majority of anaerobes were inhibited by penicillin at 3.1 mug/ml or less and by cephalothin at 12.5 mug/ml or less. Lincomycin at 6.2 mug/ml or less was active against nearly all strains. Erythromycin at a concentration of 3.1 mug/ml was active against B. fragilis; however, erythromycin was less active against the other groups. Most of the minimal inhibitory concentrations of lincomycin exceeded those of clindamycin by fourfold. Rifampin inhibited virtually all strains at 3.1 mug/ml.