Campylobacter pylori has been associated with gastritis, duodenal ulcer, gastric ulcer, and nonulcer dyspepsia. Evidence that C. pylori may be the causative agent or at least a major contributory factor in peptic ulcer disease has generated intense interest in the development of reliable methods for detecting C. pylori infections. We have developed a specific and sensitive enzyme-linked immunosorbent assay (ELISA) that detects serum immunoglobulin G antibodies directed against high molecular weight cell-associated proteins (HM-CAP) of C. pylori. In a blinded fashion we tested sera from 300 individuals and found that all of 147 HM-CAP ELISA-negative individuals were also negative for C. pylori, as documented by a negative urea breath test; also, 151 of 153 C. pylori-positive (by urea breath test) individuals were HM-CAP ELISA-positive. Campylobacter pylori was cultured from the two ELISA-negative but infected patients and these isolates did possess HM-CAP antigens, showing that these two individuals had failed to seroconvert. Thus, the specificity and positive predictive value of the HM-CAP ELISA were each 100%; the sensitivity of the assay was 98.7%, and the negative predictive value was 98.6%. The HM-CAP ELISA and the urea breath test both proved valuable for detecting C. pylori infection, the urea breath test being a more direct method whereas the ELISA is less expensive and easier to perform. Furthermore, the results of a serologic test such as the HM-CAP ELISA would not be influenced by recent ingestion of bismuth compounds or antimicrobial therapy, which might suppress C. pylori and cause a transient false-negative result in the urea breath test.