The potential in vitro synergistic effect of ciprofloxacin associated with mezlocillin, cefoxitin and clindamycin was evaluated against 30 strains of Bacteroides fragilis. All bacteria were clinical isolates, identified by gas chromatography and fermentation of carbohydrates. MICs of the drugs were determined by a miniaturized dilution broth method. Resistance or susceptibility to the drugs was not a criterion for the selection of the strains. Antibacterial combinations were tested by a checkerboard method utilizing brain hearth infusion broth and a bacterial inoculum of 10(6) CFU/ml. Microtitre plates were incubated at 37 degrees C for 48 h in anaerobic jars (BBL). Prereduced chopped meat carbohydrate broth was used to store and culture the strains. The effect of antimicrobial combinations was defined on the basis of the fractionary inhibitory concentration (FIC) indexes. We considered: synergistic effect FIC less than or equal to 0.5; additive effect FIC = 0.5-0.75; indifferent effect FIC = 0.76-2.0; antagonism FIC greater than or equal to 2. Ciprofloxacin + clindamycin showed synergistic activity against 10 strains out of 26 (38%); ciprofloxacin + mezlocillin against 9 strains out of 30 (30%); and ciprofloxacin + cefoxitin against 8 out of 29 strains (28%). Each antibiotic combination showed an additive or indifferent effect against all remaining bacteria. The synergistic effect of ciprofloxacin represents an important advantage in allowing the reduction of the dosage of associated drugs such as aminoglycosides or beta-lactams that can potentially be responsible for damage or side-effects.