The MICs of azithromycin, erythromycin and roxithromycin were determined (by an agar dilution method) for 65 strains of Gram-negative bacteria responsible for endocarditis and gastrointestinal infections, for 20 strains of non-fermenting Gram-negative bacteria and for 16 strains of anaerobic bacteria. The MICs of azithromycin were up to eight times lower than those of erythromycin and (except in the case of Flavobacterium spp.) up to 16 times lower than those of roxithromycin. Azithromycin was ineffective against strains showing a high degree of erythromycin resistance.