Seven anaerobic and facultative Gram-positive cocci and 12 clostridial species were tested for in-vitro and in-vivo susceptibilities to penicillin, clindamycin, and metronidazole, used singly or in combination with gentamicin. The in-vitro tests consisted of determination of minimal inhibitory concentration (MIC), done without or with constant amounts of gentamicin. When used alone or in combination with penicillin or metronidazole, gentamicin had negligible effects on the bacteria. When used with clindamycin, gentamicin significantly reduced the MIC for one strain each of Peptococcus magnus and Clostridium difficile. The in-vivo tests were carried out in mice and consisted of studying the bacterial contents of abscesses induced by subcutaneous injection of bacterial suspensions. Synergy between gentamicin and penicillin, clindamycin or metronidazole was shown respectively in five, three and one strain. Consistency between in-vitro and in-vivo findings was present in the above mentioned strains only between gentamicin and clindamycin. The synergy between penicillin, clindamycin or metronidazole and gentamicin in Gram-positive anaerobic and facultative organisms may have clinical implications.