1.Salmonella paratyphi Apossesses a specific Vi antigen essentially similar to that of the Vi antigen ofSalm. typhi.2. O-insensitive strains ofSalm. paratyphi Acontain more Vi antigen, and are more virulent for mice, than O-sensitive strains.3. The most suitable strains for the preparation of paratyphoid-Avaccine are those that develop both the Vi and the O antigens in maximum quantities. The methods of selecting the vaccine strain ofSalm. paratyphi Aand routinely examining the cultures are essentially the same as those applied to the virulent Vi+O form ofSalm. typhi.4. Pure A Vi antiserum is destined to assume importance in the identification ofSalm. paratyphi Astrains.5. Kauffmann's erroneous conclusion thatSalm. paratyphi Adoes not develop Vi antigen is critically reviewed.