The in vitro activity of azithromycin against human pathogenic strains of Brucella melitensis was tested at three centres and compared to that of tetracycline, the standard antibiotic currently used for the treatment of human brucellosis. MIC determination was carried out. Tested concentration ranges for both azithromycin and tetracycline were between 0.03 and 16 micrograms/ml. Brucella melitensis biotype 1, strain M16 was employed as a control microorganism. A total of 358 Brucella melitensis strains from human blood cultures were tested. MIC90 (micrograms/ml) values ranged from 0.5 to 1.00 for azithromycin and were 0.25 for tetracycline. It was concluded that there was little difference in the sensitivity of pathogenic Brucella melitensis to azithromycin and tetracycline isolated from three different regions in Spain. These results encourage further investigations on the possible therapeutic role of azithromycin in brucellosis.