Fifty-two unselected patients referred to for upper gastrointestinal endoscopy were evaluated in several ways to determine the presence of Campylobacter pylori. Antibodies against this microorganism were measured to assess the value of serology for the diagnosis of C. pylori infection. Five antral biopsy specimens were taken in each patient for culture and bacteriological determinations, histology [morphology and Warthin-Starry (WS) staining] and the urease test (2, 3 and 24 h). Serum antibodies against a sonicate of 6 strains of microorganisms were assayed by enzyme-linked immunoassay (ELISA) and an immunoblotting technique. In 14 of the 52 patients the histology of the antrum was normal, 18 patients had chronic active gastritis and 20 had chronic gastritis without polymorphonuclear infiltration. In the group with normal histology, only 1 patient was positive for C. pylori with all methods, and 1 other subject was positive for IgG and 2 for IgA only with ELISA. In the group with chronic active gastritis, 14 were positive with all methods, 1 was negative by WS only and another was negative for IgA according to ELISA, WS and antibodies. Among the patients with chronic gastritis, 7 were positive and 7 negative with all tests; in the other 6 patients the results obtained with the various tests were divergent. Four serological tests were studied and validated against culture, WS and urease test which were considered to be the reference methods. The serological tests showed high sensitivity and specificity for the detection C. pylori-associated active chronic gastritis of the antrum, and can therefore serve as noninvasive methods to identify individuals with this condition.