The administration of 2 gm. of paromomycin (Humatin) orally to human subjects was found to produce a rapid decrease in the numbers of organisms in the feces, the maximal effect occurring in 5 to 6 days. When 4 gm. was given the first day, followed by 2 gm. daily thereafter, the total flora reached a minimum in 48 to 72 hours. Regrowth of organisms during therapy occurred after 2 weeks in about 50 per cent of patients. In the others, suppression of bacteria persisted during the entire period of drug administration (4 to 10 weeks). No overgrowth of staphylococci or yeast forms occurred during or after treatment. The only untoward effect noted was diarrhea when large doses were given. Blood levels of the antibiotic could not be detected.