The susceptibility of 220 fresh clinical isolates of Bacteroidaceae, clostridia and propionibacteria to imipenem, ceftazidime, ciprofloxacin and four further substances were established by agar dilution tests. Imipenem was unique among the seven test antibiotics in that it was active against all the bacteria investigated. Whereas clindamycin suppressed all Gram-positive and 94 X 8% of the Gram-negative strains tested, and metronidazole was effective against all but the propionibacteria, ceftazidime and ciprofloxacin showed satisfactory activity against the Gram-positive bacteria only.