The minimal inhibitory concentrations of 45 antimicrobial agents were determined for 14 strains of Bacterium actinomycetem comitans (Actinobacillus actinomycetem-comitans). All the strains showed good susceptibility to tetracyclines and chloramphenicol, but not to lincomycins. Some strains were clearly resistant to penicillins, cephalosporins, and nitroimidazoles. The lowest minimum inhibitory concentrations were observed with tetracycline, rolitetracycline, methacycline, minocycline, chloramphenicol, and cotrimoxazole.