The comparative in-vitro activities of pristinamycin, its components pristinamycins I and IIA, erythromycin, clindamycin and metronidazole were studied against 174 clinical isolates of Gram-negative anaerobic bacilli and 24 strains of Gardnerella vaginalis. Susceptibilities were determined with an agar-dilution method. Against Bacteroides and Fusobacterium, pristinamycin was slightly less active than both clindamycin and metronidazole and more active than erythromycin. Against G. vaginalis, the activity of pristinamycin was similar to that of clindamycin and slightly inferior to that of erythromycin. Both pristinamycins I and IIA alone were generally inactive against the strains tested. Additionally, MICs for the B.fragilis group determined by a broth microdilution technique were one to two log2 lower than MICs obtained by agar-dilution. The bactericidal activity of pristinamycin and clindamycin was moderate and varied according to the strains tested. The chequerboard broth microdilution test against 20 strains of the B. fragilis group confirmed the synergy between pristinamycins I and IIA at every PI/PIIA ratio. Along with the activity against Neisseria gonorrhoeae, Chlamydia trachomatis, Mycoplasma and both aerobic and anaerobic Gram-positive bacteria, our results suggest that pristinamycin could be an effective drug in the treatment of gynaecological infections.