(1) A collection of S. typhi-murium strains, isolated during 11 months, in or near Manchester, was examined for the presence of symbiotic bacteriophage. In all, 177 strains derived from eighty-three incidents were investigated.
2. The different phages, or combinations of phage, allowed these strains to be divided into eleven varieties. Three hitherto undescribed types of phage were discovered.
3. With two minor exceptions, all groups of strains isolated from associated cases of infection carried the same type of phage, so that the ‘typing’ was in agreement with epidemiological expectations. In the two exceptions, the relationship of the cases was not intimate, and infection from different sources was an obvious possibility.
4. It is concluded that this is a reliable method of typing. It is probably applicable to other groups of bacteria. Its value would be enhanced by the discovery of a simpler method than filtration for sterilizing the lysato without damaging the phage.