Campylobacter antibodies of the immunoglobulin G (IgG), IgM, and IgA classes were determined by enzyme immunoassay with acid glycine extract antigen in patients and controls involved in two Campylobacter outbreaks and in 266 unselected patients with acute enteritis. The assay showed a specificity of 99% for each immunoglobulin class in sera from 200 healthy blood donors. Elevated Campylobacter antibody titers were shown in 97% of stool culture-positive patients involved in the outbreaks. Rapid changes of IgA and IgM Campylobacter antibodies were typical of the early phase of serologic response in the outbreaks and thus offered the best diagnostic value in the serologic diagnosis of acute campylobacteriosis. In unselected patients with acute enteritis, the assay revealed elevated Campylobacter antibody titers in 37 patients, of whom only 12 had had positive Campylobacter stool cultures. In the sera of patients with other bacterial findings in addition to high titers of Campylobacter antibodies, no cross-reacting antibodies were found, but there was evidence of several mixed infections.