The ability of Streptococcus diacetilactis to inhibit a variety of food spoilage organisms and pathogens in milk and broth cultures was demonstrated. Test organisms included Pseudomonas and Alcaligenes species, Salmonella, Staphylococcus aureus, Clostridium perfringens, and Vibrio parahaemolyticus. In general, approximately 99.0% and 99.9% inhibition was observed in milk and broth, respectively. Possible practical applications of the inhibition were examined. Addition of S. diacetilactis extended the shelf life of artificially contaminated cottage cheese and prevented proteolysis in milk at 7.5 C by Pseudomonas fluorescens. Staphylococcus aureus was inhibited greater than 99% in vanilla cream filling, ham sandwich spread, chicken gravy, soy milk, and ground beef stored at 25 C for 24 hr. Development of the gram-negative flora of ground beef was also inhibited greater than 99% after storage at 7.5 C for 7 days. Possible roles for several factors in the mechanism of inhibition by S. diacetilactis are briefly discussed. The effects of pH reached and acids produced by S. diacetilactis on the growth of S. aureus are described. A greater role for the lactic acid bacteria in fermentations in the food industry is suggested.