Virulence plasmids, which are found in a small number of Salmonella serotypes, greatly enhance the extraintestinal growth of salmonellae and lower the LD50 by 2-5 logs in experimental murine infections. To determine if virulence plasmids are important in the pathogenesis of Salmonella bacteremia in humans, blood and fecal isolates of Salmonella typhimurium from California were examined for the presence of a virulence plasmid. Colony blots were done using a labeled probe made from the highly conserved EcoRI fragment of the Salmonella dublin virulence plasmid. A total of 42% of the fecal and 76% of the blood isolates hybridized with the probe. This is the first evidence that the virulence plasmid is important in the pathogenesis of Salmonella bacteremia in humans.