UNTIL a few years ago, the beneficial effects resulting from the ingestion of the Lactobacillus acidophilus in cases of gastro-intestinal disturbances were attributed to the simple overgrowth of the offending pathogen by the Lactobacilli. Recently a number of workers have indicated that there was more to the reaction than just competition between strains. The substance produced by L. acidophilus has been called variously antagonist1, lactobacillin2, lactocidin3, and antibiotic4. All these workers found that the substance was extremely labile, and attempts to isolate it met with failure. A successful method for the visual demonstration of the phenomenon is described here but isolation has not yet been successfully accomplished.