A study was made of the immunological properties of the Vi antigen obtained from Escherichia coli 5396/38 by a NaCl-ethanol fractionation procedure. The purified Vi antigen could be absorbed on collodion particles or on erythrocytes which then became specifically agglutinable by Vi antisera. It possessed excellent serological activity and specificity in these tests and also in the quantitative complement-fixation and quantitative precipitin tests. In the range of antibody excess the Vi antigen precipitated in a uniform manner the Vi antibody in coli, ballerup, typhoid and purified Vi-antisera. Supernatant fluid analysis in the quantitative precipitin test with E. coli antiserum indicated that the purified antigen approached but did not quite meet this criterion of immunological homogeneity. The antigen was markedly effective in the protection of mice against challenge with virulent strains of Salmonella typhosa. It was antigenic for mice, rabbits and man, evoking the production of antibody reactive with both Vi antigen and with Vi strains of S. typhosa. The antibody produced in rabbits possessed mouse protective potency and in vitro activity essentially comparable to the Vi antibody obtained by immunization with Vi strains of S. typhosa, although certain differences in these activities were pointed out. The antigen was as effective as V form S. typhosa in the removal of mouse protective potency from various Vi antisera.