The presence, in the cheese milk, of phage active against the organisms of the ‘singlestrain ’ culture in use as starter leads to its multiplication during the cheese-making process. The extent to which the presence of phage is made evident in the vat depends largely upon the amount of initial infection. A heavy infection permits sufficient multiplication of the phage to cause lysis of the organisms and consequent cessation of acid development before the manufacturing process is completed. A light infection may have no noticeable influence upon the performance of the starter in the vat. Intermediate amounts have an effect on the rate of acid development in the later stages of the process, but within certain limits there is little adverse influence on the quality of the cheese produced.