Addition of lactic acid bacteria to refrigerated raw milk inhibited growth of Gram‐negative psychrotrophic bacteria. In order to obtain complete inhibition, population levels of lactic acid bacteria approaching 1× 108cfu/ml were required. At this inoculum level a substantial pH change was observed in the milk during storage. The degree of inhibition was related to the final pH of the milk. Inhibition of the psychrotrophs was brought about by adjustment of pH with L(+) lactic acid but not by HCI. Inhibition by the lactic acid bacteria was observed in heat‐treated milks seeded with a small volume of raw milk, but no inhibition of Gram‐negative psychrotrophic growth was observed when cell‐free preparations of lactic acid bacteria were used. A cell‐free ultrafiltrate of one lactic acid culture did inhibit a strain of Bacillus