Summary and conclusions: 1. Two groups of typhi-phages, one lysing typhoid bacilli only, the other Gaertner bacilli also, have been studied. Each phage stimulated, in Salmonella typhi, resistance to all phages of its own group but none to those of the opposing group.2. It is probable from the range of their activity, that typhi-phages of the second group attack the receptor IX (of the Kauffmann-White system of Salmonella antigens).3. There are strong grounds for believing that the phages of the first group, the specific typhi-plages, attack the Vi-antigen of Felix, for(a) typhoid cultures which are readily agglutinated by Uaertner antiserum are relatively or entirely insensitive to these specific typhi-phages,(b) the specific typhi-phages transform O-inagglutinable cultures into O-agglutinable forms, and(c) the Vi-antigen is the only known element of the typhoid bacillus which is peculiar to that organism and so corresponds in specificity with the phage.4. The fact that resistance acquired against a particular bacteriophage does not prevail against all other bacteriophages is not to be ascribed to the quantitatively differing potency of phages as suggested by d'Hérelle but to their qualitatively different activities. Each type of phage has its particular “point of attack” in the bacterium and the distinct “points of attack” of different phages develop their resistance independently. It seems that, as Marcuse has urged, the “points of attack” are located in the bacterial receptors.