Outbreaks of salmonellosis due to Salmonella typhimurium involving cows and calves on four farms and humans on two of the farms in one district in Denmark are described and follow-up studies are reported. Up to 75% of the cows at one farm developed clinical salmonellosis and two animals died. On another farm 25% of the cows suffered from the disease and there were two human cases of salmonellosis due to S. typhimurium. On three of the farms, a number of the recovered animals excreted salmonellae for one to three months post treatment. Biochemical reactions showed that outbreaks at two farms were due to the same biotype, while at the third farm the biotype was different. Tetracycline resistance was demonstrated in strains from one farm. The serum agglutination test was found to be rather insensitive and non-specific for the detection of recovered and excretor animals. The source of infection was difficult to establish, except for one farm where S. typhimurium was isolated from a sample of the feed. On one of the farms, the owner was strongly suspected of having spread the infection through his herd. The significance of the findings and ways to prevent spread of infection are discussed.