The susceptibility of 54 clinical isolates of Acinetobacter calcoaceticus ss. anitratus to 16 antimicrobial agents was determined in vitro with inoculum of 10(4) and 10(6) cfu by a standard agar dilution method. The most active agents were imipenem, SCH 34343, ciprofloxacin, difloxacin (A-56619), and A-56620. Only imipenem and Abbott quinolones (A-56619 and A-56620) remained active when tested with the heavier inoculum. Except for ticarcillin and ceftazidime, which showed only moderate activity, the extended-spectrum penicillins and cephalosporins, as well as aztreonam and aminoglycosides, were inactive against these highly resistant strains. Nine isolates were selected for combination studies of ciprofloxacin with seven beta-lactams and three aminoglycosides using a checkerboard agar dilution technique. Synergistic or additive interactions at clinically achievable concentrations were more common with amikacin (eight isolates), tobramycin (seven), ceftazidime (six), cefoperazone (six), and aztreonam (six), than with other agents, including mezlocillin (four), piperacillin (three), gentamicin (two), and cefsulodin (two). Antagonism was rare, only occurring with mezlocillin in a single strain. These data suggest that combinations of ciprofloxacin with these agents may be useful for some nosocomial multidrug resistant A. calcoaceticus ss. anitratus infections.