The serologic responses to Campylobacter jejuni in persons involved in two clusters of infection and in control subjects were studied. In the first cluster, in which previously unexposed persons drank raw milk, the attack rate was high and elevated complement-fixing (CF) and specific IgG and IgM antibodies were demonstrated. In the second cluster, involving farmers who chronically drank raw milk, the attack rate was low, but titers of CF and IgG antibodies were high in both affected and unaffected persons. At a control dairy farm, where raw milk was drunk regularly, asymptomatic infection and high CF titers were demonstrated. In contrast to the findings in the first cluster, the titers of IgM antibody among the dairy farmers were low. These studies suggest that chronic exposure to C. jejuni may lead to immunity that may possibly be mediated by IgG.