The in vitro activity of 21 antimicrobial agents against 143 Acinetobacter strains (belonging to 12 different DNA hybridization groups) was studied. Of the antibiotics tested, ciprofloxacin, ofloxacin, doxycycline and imipenem had the lowest MICs. For most antibiotics there was a considerable overlap of the MICs between the DNA groups; however, some differences in antimicrobial susceptibility between DNA groups were found. Strains of DNA groups 2 (A. baumannii) and 13 (unnamed) were most resistant, and strains of DNA groups 8 (A. lwoffii) and 5 (A. junii) less resistant. Many DNA groups contain both saccharolytic and non-saccharolytic strains but in the present study no differences in antimicrobial susceptibility were found between strains of a particular DNA group with regard to this property.