Bacteriophages for lactic streptococci were found to occur in the atmosphere of commercial cheese factories. Finely divided particles of whey emitted from the whey separator appeared to be the main vehicle for the air-borne phage although whey-contaminated dust probably also played a part. The concentration of air-borne phage was sometimes so great that, with the use of a normal bacteriological aseptic technique, it was impossible to prevent infection of the starter cultures for more than a few propagations. Protection of the starter from air-borne phage eliminated phage failures.

The question of the origin of bacteriophage is discussed in the light of the recognition of the prevalence of air-borne phage. The occurrence of air-borne infection could account for most of the instances of starter failures which were previously thought to be due to spontaneous phage development within cultures.