The ability of Salmonella enterica to survive an incubation of 2 h in fresh, pooled guinea-pig serum was determined comparing strain sets of serovariants Dublin and Typhimurium, harbouring or lacking the virulence plasmid. All strains showed marked serum resistance, which was slightly decreased among cured strains of Typhimurium. However, when introduced into a rough Escherichia coli strain, all Typhimurium plasmids substantially increased the resistance of the host strain to guinea-pig serum, whereas the Dublin plasmid did not. The traT gene, previously shown to affect serum resistance, was identified on all Typhimurium plasmids, but not on the Dublin plasmid. Mutational inactivation of the traT gene on the Typhimurium plasmids eliminated the serum resistance mediated by the plasmids.