New macrolide antibiotic, azithromycin (AZM), was administered to six healthy male volunteers and its effects on their intestinal microflora were investigated. Each volunteer was given 500 mg of AZM orally, once a day for 3 consecutive days. Stool samples were obtained from them prior to the medication and 1, 7, 14 and 28 days after the third day the medication. A slight decrease in the total aerobic bacterial count was observed. Also, several species of anaerobic bacteria showed slight decreases through the 14th day post medication. Individual variances were observed, however. A marked decrease in the number of Bifidobacterium was found for each of the volunteers. Clostridium difficile was detected from one of the volunteers on the 28th day post medication without diarrhea related symptoms.