A study of the inhibition of the growth of Streptococcus cremoris 972 by the enzyme lactoperoxidase has shown, in agreement with previous investigations, that the inhibition requires a source of both peroxide and thiocyanate. The thiocyanate may play more than one role. It stabilizes the very dilute solutions of lactoperoxidase employed in these studies, and its oxidation products may be involved in the inhibition. Binding of the enzyme by the microorganism is suggested by the fact that when the organism was preincubated with the enzyme and then in a medium free from the enzyme, but containing peroxide and thiocyanate, the growth of the organism was inhibited. This inhibition has all the properties of the enzyme-containing system. Although no dialyzable factor could be demonstrated to cause the inhibition, the inhibitory state involving peroxide, the enzyme, and thiocyanate survived for at least 60 min before cells were added to the medium. When catalase was present in the medium prior to the addition of the cells, the inhibition was completely reversed. It was only partially reversed if catalase was added a few moments after the addition of the cells. The data have been interpreted as indicating that the inhibition takes place rapidly and requires the formation of a quaternary complex of the cells, thiocyanate, peroxide, and the enzyme lactoperoxidase.