1. The low Vi-antigen content and low mouse virulence of freshly isolated cultures of the Oswestry strain of Salm. typhi reflected its low virulence for man. This is additional evidence of the association between the degree of virulence of Salm. typhi and its Vi-antigen content.
2. The prediction that the epidemic caused by this typhoid strain of low mouse virulence would resemble a paratyphoid-B outbreak more closely than an average typhoid outbreak proved to be correct.
3. Sera from convalescent patients in this typhoid outbreak showed significant Vi-agglutinin titres in spite of the low Vi-antigen content of freshly isolated cultures.
4. The nutritional requirements of the Oswestry strain were unusual, and its sensitivity to the specific Vi-typing phages was variable. This caused considerable difficulty in the typing of the cultures.
5. Additional tests of identity had to be employed in order to distinguish the epidemic strain from other strains of the same Vi-phage type isolated in the vicinity. Colony size, colonial morphology, biochemical reactions and sensitivity to anti-O bacteriophage served as useful ancillary criteria.