Within the past few years, the in vitro antimicrobial susceptibility pattern of anaerobic bacteria has undergone a gradual change (1,14,21). For example, some strains of Bacteroides melaninogenicus, other Bacteroides species, and some Clostridium species have become moderately resistant to penicillin G (7,9). Also, many strains of Bacteroides fragilis and other anaerobes are now resistant to ... [Show full abstract] tetracycline (21), and in 1976, two strains of Bacteroides fragilis subsp. ovatus (18) resistant to clindamycin were reported. Sutter and Finegold have suggested that specialized centers, nationwide, perform periodic antimicrobial susceptibility tests to monitor changing patterns of resistance of anaerobic bacteria to antimicrobial agents commonly recommended in the treatment of patients with anaerobic infections, and to determine the activity of newer agents against anaerobic bacteria (21). The purpose of this study was to assess the in vitro activity of several antimicrobial agents currently used for therapy of anaerobic infections and to explore the activity of several newer drugs against a variety of recent clinical isolates of anaerobic bacteria.