Serial measurements of serum and secretory antibodies to Salmonella typhimurium were made by ELISA in eight patients with suspected reactive arthritis identified after a large outbreak of Salmonella gastroenteritis. All three patients from whom Salmonella had been isolated developed significant serum IgG, IgA and IgM antibody responses. Only one of the three possessed HLA-B27. A further three patients, two with HLA-B27, had raised antibodies, although none had experienced gastroenteritis. Salmonella infection was not confirmed in the remaining two patients. The three B27-negative patients with confirmed reactive arthritis had HLA-B locus antigens which serologically cross-react with B27. One of six patients with confirmed reactive arthritis was under the age of 25 years whereas 256 of 418 (61%) patients with uncomplicated enteritis were under this age. The development of reactive arthritis may follow subclinical Salmonella infection and is influenced by genetic and age-related factors.