Antibody response to Campylobacter jejuni/coli (CJC) was investigated, using an enzyme-linked immunosorbent assay, ELISA. With a mixture of lipopolysaccharide from two CJC strains as antigen in ELISA, all 24 tested rabbit anti-CJC sera showed high antibody levels. However, only 70% of sera from patients with Campylobacter enteritis demonstrated an antibody response against the combined LPS antigen, using paired sera. In addition, the results obtained suggested non-specific binding of human immunoglobulin. When 24 formalinized whole CJC bacteria were used as antigen in ELISA, all corresponding rabbit antisera reacted with one strain (M 14). Essentially no unspecific binding of human immunoglobulin was obtained. Antibodies were detected in sera from healthy blood donors and at a lower level in sera from children, suggesting early immunization. In 67 enteritis patients with positive stool cultures for CJC, a significantly increased level of IgG antibodies could be detected in single or paired serum samples from 82% of the patients. An IgG titre increase occurred early in the course of infection, suggesting a boosting of an earlier immunization. IgM antibodies could be detected in the same sera in 77% of the patients. Considering both IgG and IgM analyses of the enteritis sera, 94% of the patients were positive in Campylobacter ELISA serology compared with only 5% of healthy controls.