STRAINS of lactobacilli were recently isolated from Gruyère cheese which gave zones of inhibition against Staphylococcus aureus 20-30 mm. in diameter. This inhibition appeared to be antibiotic in nature, perhaps similar to that described by Grossowics, Kaplan and Schneerson1, and the active principle was called `lactobacillin'2. The tests were done on agar buffered to pH 7.0 with 1 per cent phosphates to avoid false positive effects due to lactic acid2,3. Ritter has described similar experiments4. His strains were isolated in Switzerland from Emmenthal cheese and through his kindness they were included in this investigation. Most of our cultures have now been identified serologically by Dr. M. E. Sharpe as Lactobacillus lactis.