Antibiotic-induced changes in the fecal microflora after oral administration of tetracycline hydrochloride and doxycycline for 8 to 10 days were compared. A significant difference was noted in the concentrations of Escherichia coli resistant to tetracyclines. With tetracycline hydrochloride, there was a mean increase of approximately 10 4 resistant strains per g compared to only 10 1 /g for doxycycline. This difference is ascribed to reduced intestinal concentrations of bioactive drug with recommended oral dosage for doxycycline.