A total of 1158 sera were tested by an enzyme linked immunosorbent assay (ELISA) for IgG and IgA antibodies to Campylobacter pyloridis. When used to test sera from gastroendoscopy patients this method had a sensitivity of 91% and a specificity of 75%. Ninety-eight of 108 infected patients (91%) were seropositive whereas only 23 of 92 (25%) uninfected patients were seropositive (p less than 0.001). Positive serology was also associated with histological gastritis (p less than 0.001). When sera from occupational groups were compared to random blood donor sera, meat workers, freezing works veterinary surgeons and pest control officers had significantly higher seropositivity. Cook Island (33 of 85 (39%)), Samoan (57 of 129 (44%)) and Tongan (39 of 56 (70%)) blood donors also had greater seropositivity than random blood donors (15 of 102 (15%)), (p less than 0.001 for all three ethnic groups). Seropositivity increased with age. The results indicate that C pyloridis infection is common among certain groups within New Zealand. The implication of this finding with respect to the hypothesis that C pyloridis is aetiologically related to dyspepsia and peptic ulcer disease is discussed.