Meropenem was compared with imipenem and nine other antimicrobial agents, against 101 strains of the Bacteroides fragilis group. Meropenem was active against all strains tested, and its activity was similar to, and in many cases better than, that of imipenem. The activity of meropenem was similar to that of metronidazole, and greater than that of the other antimicrobial agents tested. The bactericidal activity of meropenem against B. fragilis was impressive, since the MBC to MIC ratios were no greater than two. The bactericidal activity was confirmed by time-killing curve assays with two strains which showed that meropenem was rapidly bactericidal and reduced the initial inoculum significantly during the first 4-6 h. The postantibiotic effect of meropenem (2-4 h) and a sub-inhibitory concentration of 1/4 x MIC suggested that meropenem interferes with the normal growth of B. fragilis, even when administered concentrations fall below the MIC. MICs of meropenem were affected minimally by the pH of the medium or by an increase in inoculum size. Meropenem continued to have good activity against a B. fragilis strain that had been induced for the production of cephalosporinase. The in-vitro data presented in this paper indicate that meropenem is a promising antimicrobial agent which may be useful in the treatment of problematic mixed infections.