The microtiter broth dilution method was employed to determine the in vitro susceptibility of 525 recent clinical isolates of anaerobic bacteria to sodium fusidate. The minimal inhibitory concentrations of sodium fusidate ranged from </=0.06 to 1.0 mug/ml for 155 strains of anaerobic gram-positive rods and 130 strains of anaerobic gram-positive cocci. Minimal inhibitory concentrations ranging from </=0.06 to 32 mug/ml were observed for 240 strains of anaerobic gram-negative rods. Among the latter group a minimal inhibitory concentration of 16 mug/ml or greater was encountered with 16% of 45 Bacteroides fragilis strains, 19% of 32 Bacteroides thetaiotaomicron, and all 7 strains of Fusobacterium necrophorum. Minimal inhibitory concentrations for Veillonella parvula, the only gram-negative coccus tested, ranged from 0.5 to 8.0 mug/ml.