1. Investigations have been carried out on the bacteriophage method of typing staphylococci developed by Wilson & Atkinson (1945).

2. Acquired phage resistance has been shown to be responsible for the classification of many strains as different phage types.

3. Repeated testing of the same strains showed that the phage types were stable in character when the strains were maintained under laboratory conditions.

4. When two strains of staphylococci, one of which was lysogenic to the other, were grown together in broth, the susceptible strain underwent a change in phage type. As a result of this, it is considered that, in epidemiological studies, infective material should be plated directly on to a solid medium and not incubated first in broth, as is sometimes done. Evidence has been obtained suggesting that this change of type may also take place in the field. This phenomenon may be of some significance in epidemiological studies extending over a long period of time.

5. An attempt has been made to classify staphylococcal strains by phage methods into types that differ other than by virtue of acquired phage resistance. These are called, for convenience, genetic types. Reasons are given for the view that this may not be a satisfactory method of classification in practice.