The immune response was studied in 238 human patients with Campylobacter jejuni/coli (CJC)-infections in Rotterdam by the counterimmunoelectrophoresis (CIE) test, a commercial complement fixation test (CFT) and the passive haemagglutination test (HA). Antibodies became detectable in the three tests around 4 days after the onset of complaints. Between the 7th and the 20th days after onset of illness 79%, 80% and 53% of the patients demonstrated antibodies by the CIE, the CFT and the HA, respectively. The HA took 30 days to reach 60% positive serum samples and this percentage declined to 35 by the 50th day. Antibody titres demonstrated in the CIE and the CFT declined more slowly. CIE and CFT performed with antigens from a limited number of heat-stable serotypes can be used in the evaluation of the humoral immune response in CJC-infections.