A "milk-peroxidase" agar was prepared to select cultures of lactobacilli that most actively produce hydrogen peroxide. O-tolidine and peroxidase contained in the medium reacted with hydrogen peroxide produced by the bacteria to form a colored zone around the colonies during their formation on the agar at 37 C. There appeared to be no relationship between the size of zones surrounding the colonies on the agar medium and the amount of peroxide produced in milk at 5.5 C. Cells from frozen concentrated cultures of selected lactobacilli were added to milk to achieve desired populations in the interaction experiments. A significant inhibition of a psychrotrophic culture in autoclaved milk at 5.5 C occurred when at least 2.5 × 107 lactobacilli were added per ml. Larger numbers of lactobacilli resulted in greater inhibition of the psychrotroph. The intensity of inhibition of a psychrotrophic culture in autoclaved milk varied among the cultures of lactobacilli tested. In most instances, the more active inhibitory cultures produced more peroxide in refrigerated milk than less inhibitory ones. In preliminary experiments, a culture of Lactobacillus bulgaricus selected by the above procedure produced no inhibitory effect against the growth of psychrotrophic bacteria in non-agitated raw milk at 5.5 C.