The proportion of disease outbreaks associated with Salmonella dublin has declined during the last 15 years, but the organism remains an important cause of salmonellosis in cattle. In a survey of 104 farms at which enteritis in calves was diagnosed, salmonellas were isolated from 19 and were associated with diarrhoea in 12% of calves from 24% of outbreaks. One hundred and forty seven strains of S. dublin were characterised on the basis of morphology and biochemistry. The serotype is distinguished by a requirement for nicotinic acid and the possession of a 'serotype-specific' plasmid of approximately 50Mdal. The serotype represents a homogeneous grouping and it was not possible to produce a biotyping scheme superior to that of Walton (1972), although the strains could satisfactorily be subdivided into 33 phage-types. The virulence of strains as determined by their oral LD50 in C57 mice ranged from approximately 102 to greater than 108 although the majority of strains had LD50 values below 104. Virulence was not usually affected by culture on laboratory media but its measurement was highly dependent upon the method of oral inoculation. Many of the strains which were 'avirulent' or of 'reduced virulence' in mice differed from 'virulent' strains in several characteristics. However lack of virulence was usually associated with strains which were rough or 'semi-rough', anaerogenic, slow growing, or without mannose-sensitive and mannose-resistant haemagglutinins or 'serotype-specific' plasmids. There was a good but not perfect correlation between virulence for mice and virulence for rats and calves. Strains which lacked a 'serotype-specific' plasmid were 'avirulent' in mice following oral or intraperitoneal infection but retained residual virulence when inoculated in rats intraperitoneally or in calves orally. Calves were protected against S. typhimuriurn and S. dublin infection by the colostrum of cows vaccinated with formalinised S. typhimuriurn.