Cultures of lactic streptococci partially resistant against a phage race and growing in regular association with that race are, as a result of the presence of the phage, protected from attack by some other races of phage. Such phage-carrying cultures were tried as cheese starters in commercial factories under conditions where no protection for the starter against air-borne phage was given. The results were quite satisfactory for several months, but eventually there appeared in the commercial factories races of phage which would attack the phage-carrying culture and thus depress or completely check acid development in the cheese vats. It seems likely that the large number of phage races which evidently exist in nature would make a complete protection along these lines very difficult to accomplish.