The antibody response to Campylobacter jejuni/coli infection was investigated in 59 patients involved in two outbreaks of milk-borne infection and in sporadic infections in the community. Agglutinins and complement-fixing (CF) antibodies were detected in nearly all these patients. Agglutinins were present in 25% of normal sera at low titres (not greater than 1/160) but CF antibody titres of 1/4 or 1/8 were present in only 2.0%. The agglutination reactions in convalescent sera were best developed with the homologous or an antigenically similar strain whereas the CF test, with sonicated organisms as antigen, was less strain-specific and was more suitable as a routine test. Antibody was present seven to 10 days after the onset of symptoms.