RESUMEN
The authors have compared the biochemical, immunochemical and immunobiological properties of a chloramphenicol-resistant S. typhi strain isolated during the typhoid fever epidemic in Mexico of 1972 and those of a chloramphenicol-sensitive strain (S. typhi Ty2). They have found no difference in the chemical composition of the two strains. The immunobiological investigations have shown, that the chloramphenicol-resistant strain contains less Vi antigen, it is less virulent on mice, and its active and passive mouse-protective ability is lower than that of the chloramphenicol-sensitive S. typhi Ty2 strain. These findings supported by the result of the electrophoretic analysis suggest that the chloramphenicol-resistant strain is a VW strain, while the chloramphenicol-sensitive strain is a full V strain; the quantitative difference found between the immunogenicity of the two strains to the advantage of the chloramphenicol-sensitive strain may probably be explained by this fact. On the basis of their immunological investigations the authors are of the opinion, that vaccines prepared from chloramphenicol-sensitive S. typhi strains (provided these are full V strains, as e.g. the S. typhi Ty2 strain) will protect most likely also man against infection due to chloramphenicol-resistant S. typhi strains.