Laboratory animals are not sensitive to human S. typhi strain (O+ Vi+). But the chick embryo is very susceptible to infection by this strain. This property can be used to test protective activity of immuno sera. Several antigenic fractions have been prepared from Ty6S strain (0- Vi+) by washing methods. The " outer layer " of the bacteria is easily removed by NaCl 0,5 M. Injected in rabbits this preparation produces the synthesis of protective antibodies. The active antigen X is spun down with the Vi antigen. The relation of X antigen to Vi antigen is being studied.