The virulence-associated plasmids of strains of Salmonella gallinarum and S. pullorum were transferred separately by mobilization with the F plasmid into virulence plasmid-cured derivatives of S. gallinarum, S. pullorum, and S. typhimurium and into a prototrophic Escherichia coli K-12 strain. The transconjugants were tested for virulence in chickens of different ages and in mice. The S. gallinarum and S. pullorum plasmids were able to restore full virulence in the three Salmonella strains, thus demonstrating functional homology in virulence plasmids from these Salmonella serotypes and biotypes. The virulence phenotypes of the transconjugants remained the same as that of the parent strain of the recipient. This, together with the fact that E. coli K-12 containing either of the virulence plasmids was avirulent for chickens, suggested that in addition to plasmid genes, chromosomal genes are important in determining virulence, particularly in determining the ability to survive and multiply in the cells of the reticuloendothelial system. The virulence plasmids were not self-transmissible and could not be transduced by temperate bacteriophages lysogenizing field strains of S. gallinarum. They were not in the same incompatibility group as F but were fi+.