A monoclonal antibody, CP11, has been produced which is directed against the ureas of Campylobacter pylori. This antibody has been used to look for antigenic cross-reactivity, in other ureolytic and non-ureolytic campylobacters, by immunohistological techniques. It has also been used to investigate the helical-shaped organisms found in the stomach of the human, monkey and cat (CS1) and the ileum of the rat (ST1). Interestingly the antibody cross-reacted with the gastric helical organisms from the human, monkey and cat but not with the rat helical organism. No cross-reactivity was observed with C. mustelae or the other ureolytic campylobacters, C. nitrofigilis and the urease positive thermophilic campylobacters. These results are discussed in relation to the phylotaxonomy of these organisms.