1. Antisera prepared in rabbits or calves against live Salmonella dublin gave mice some degree of protection against oral infection with this organism. Both antiserum prepared against heat-killed S. dublin and dead vaccines prepared in a variety of ways produced little or no immunity.

2. A rough variant of S. dublin of low virulence for mice, no. 17 A, produced a reasonably good immunity against oral infection with S. dublin in mice but not in calves. Mice that survived injection with another rough variant that possessed a considerable degree of virulence for these animals, no. 51, were immune to oral infection with S. dublin. Experimentally and naturally, this variant and, to a lesser extent, 9S, a smooth variant of S. gallinarum of reduced virulence, produced an appreciable degree of immunity in calves against S. dublin infection; none of the calves injected with these variants showed any signs of ill-health as a result.

3. Two rough variants of S. cholerae-suis, nos. 3 and 6, possessed a considerable degree of virulence for mice; those that survived were resistant to oral infection with S. cholerae-suis. Experimentally and naturally, both variants produced an appreciable degree of immunity in pigs.

4. Mice that survived vaccination with the rough S. cholerae-suis variant no. 6 were resistant to oral infection with S. dublin. Those that survived vaccination with the rough S. dublin variant no. 51 were resistant to oral infection with S. cholerae-suis and S. typhi-murium; they were fully susceptible to parenteral administration of Escherichia coli and Erysipelothrix rhusiopathiae.

5. Vaccination with S. cholerae-suis variants 3 and 6 and S. dublin variant 51 provoked the formation of H but not O antibodies. These variants were never found to mutate from rough to smooth in vitro or in vivo.