Monoclonal antibodies were raised against Campylobacter pylori to detect the organism in clinical specimens by the immunoperoxidase technique. Twenty-one hybridoma cultures obtained were screened on tissue sections of gastric mucosa to identify antibodies that did not cross-react with host cells. To select antibodies with high grade of specificity to C. pylori, hybridomas were also screened by an enzyme-linked immunosorbent assay against a wide panel of gram-negative bacteria. Fifteen antibodies showed a variable grade of cross-reactivity with gastric mucosa, indicating the presence of at least three epitopes shared by C. pylori and epithelial cells. Three of the six antibodies non-reacting with gastric mucosa (CB-1, CB-26, and CB-31) also were found to be specific for C. pylori. Using CB-26 in an indirect immunoperoxidase test, gastric brushings from 60 patients consecutively undergoing endoscopic examination were examined. By light microscopy, we observed the presence of large amounts of C. pylori in smears from all the 32 patients with positive culture. A small number of bacterial cells were also seen in 8 of 28 patients whose cultures were negative. These findings suggest that the monoclonal antibody CB-26 might be of value for rapid and specific detection of C. pylori.