Seventy-two field strains of smooth Salmonella cholerae-suis var kunzendorf were examined for susceptibility to killing by antibody-complement (Ab-C) after they were exposed to Tris-EDTA, lysozyme, or saline solution. Considerable differences were found among the strains with some being susceptible to killing under all conditions and some being competely resistant. Evidence was obtained to indicate that Tris-EDTA may inhibit the growth of S. cholerae-suis in addition to its documented effect on the release of lipopolysaccharide from the cell wall of gram-negative bacteria. Twenty-two of these strains were further evaluated for their susceptibility to killing by porcine peripheral blood polymorphonuclear neutrophils (PMNs) under various conditions. Susceptibility to PMN killing was found to correlate well with the Ab-C susceptibility of these strains. The mouse virulence of these strains was examined. Eight of the strains were selected for further evaluation in the pig. With one exception, the virulence of these strains for pigs was well predicted by the results of the in vitro Ab-C bacteriolysis. Mouse virulence was also found to correlate with pig virulence but was somewhat less accurate. The immune response of the pig to one of the strains of S. cholerae-suis was evaluated with and without the presence of the immunosuppressive drug cyclophosphamide. In addition the effects of CY on the delayed hypersensitivity (DH) reaction to Mycobacterium avium were examined. For this, pigs were sensitized with M. avium sensitinogen. Pigs given 20 mg/kg of CY on days 0, 2 and 4 and infected with 10('6) S. cholerae-suis on day 0 were more severely affected than those given the same dose of CY and infected on day 4. The DH response in the former group was equivalent to the nonsensitized group and thus was severely depressed. The DH reaction of pigs infected on day 4 was also decreased compared to the sensitized controls. Mortality rates and serological responses were compared for the various groups of pigs. The virulence and immunogenicity of several mutants of S. cholerae-suis were evaluated in mice. An aro('-) galE mutant of strain 38 proved to be reduced in virulence from the parent strain but provided only moderate protection against homologous challenge. Thymidine-requiring mutants of strains 9, 33, 38, 51 and 61 were observed to be avirulent for mice. All provided as much or more protection against challenge than the parent strain.