Antibody to Campylobacter pyloridis was measured by ELISA in the sera of 160 patients from whom gastric biopsy specimens were also obtained. The antigen was an acid-glycine extract of C. pyloridis, and titers ranged from 80 to 22,000 ELISA units (EU). Of 117 patients in whom C. pyloridis was detected microbiologically or histologically, 87 (74%) had a titer greater than or equal to 300 EU, and only one had a titer less than 150 EU. Of 43 patients in whom C. pyloridis was not detected, only two (5%) had a titer greater than 300 EU. Thus, for a titer of 300 EU the ELISA test had a specificity of 97% and a sensitivity of 81%. At 150 EU the specificity was 78%, and the sensitivity was 99%. Histological diagnosis of active chronic gastritis was associated with a high median ELISA titer (485 E), chronic gastritis with a much lower titer (150 EU), and normal histology with a titer of 110 EU. Discriminating use of this serological test could be of assistance to detect C. pyloridis in the gastric mucosa.