1. Previous work on bacteriophage infection of starters as a cause of slow acid development in cheese manufacture is reviewed. Air-borne phage seems to be the form of infection which is most insidious and most difficult to exclude. Hypotheses are developed to explain the sequence of events observed in commercial practice. The spread of phages as dust or droplet infections seems to explain the gradual increase of trouble with a given starter culture within a single factory and also throughout an entire dairying district.

2. When phage is established in the surroundings of a dairy factory a cycle of infection can develop. Besides the air-borne droplet infection there may be daily reinfection of cheese milk from incompletely sterilized factory and farm equipment which comes into contact with the milk. A description is given of methods which have been used to determine in any given case whether slow acid development is due to phage infection of the starter itself or to infection of the milk from factory or farm utensils.

3. The problem of preventing phage infection of the starter culture is fully discussed. The extent of the air-borne infection to be guarded against is indicated. The equipment which has so far been developed in New Zealand for the aseptic handling of starter cultures is described in detail. The most promising lines of development are indicated.