A strain of Bacillus licheniformis, established in the digestive tract of gnotobiotic mice, inhibited the subsequent establishment of a Clostridium perfringens strain ingested by the animals. This inhibitory effect depended on the in vivo production by B. licheniformis of an antibiotic substance having a number of the characteristics of bacitracin. If C. perfringens was the first to become established in the digestive tract of the gnotobiotic mice, B. licheniformis also became established but did not produce any antibiotic. Mutants of C. perfringens resistant to the antibiotic substance were not observed when the antibiotic was produced in situ by B. licheniformis, but were rapidly selected when the Bacillus culture filtrate or bacitracin was administered per os. B. licheniformis was also capable of eliminating from the digestive tract 5 of the 13 additional bacterial strains tested.