SUMMARY: The identification of the type A phages of has hitherto been based on antigenic structure, host range, and plaque characters. The discovery of an apparently phage-free strain of , equally sensitive to all these phages, has enabled us to prepare standardized lysogenic bacteria, alike in all respects except in the symbiotic phage (prophage) they carry. With these it has been possible to test cross-immunity with a high degree of accuracy. The immunity pattern of a collection of 598 type A phages has been examined, and subdivisions in the original types have been revealed. Twelve types are now identified. The test is described and the significance of the findings is discussed.


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