The present study demonstrates that vaccines prepared from smooth V form S. typhosa killed with formalin, which has little effect on the Vi antigen, are more effective in the production of active immunity to experimental typhoid infection in mice than are organisms from the same culture killed with heat, which destroys the Vi antigen. The reduction in immunizing potency associated with heating was accompanied by a reduction in the agglutinability of the vaccine in specific Vi immune serum, and an increase in specific 0 agglutinability. The demonstration of differences in active immunizing potency was made possible by the use of techniques of assay similar to those used by Batson.

The demonstration of difference between the coefficients of regression for the two vaccines is presumptive evidence that the mechanisms of action of the two vaccines were qualitatively different. Qualitative differences could have arisen in the vaccines from antigenic differences produced by the methods of killing. However, the greater bacterial content of the E. D. 50 of the heat-killed vaccine could have been responsible for this effect. The factors could have acted alone or together to produce the effect, but we think that the possibility mentioned first is of greater importance.

It was found that mice immunized with heated or unheated vaccines were not sensitive to variations in challenge doses of the magnitudes involved in this experiment. The variations in challenge dose did not influence the reliability of the comparison of immunizing doses. This observation partially confirms similar observations made by Batson.

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