118 patients with chronic gastritis were the subject of the tests; they were divided into two groups on the basis of a bacteriological (culture) diagnosis of the infections by Helicobacter pylori: group I-patients with the infection present only in the stomach; group II-patients with the infection synchronically occurring in the mouth and the stomach. Both groups underwent the 14C-urea breath test to detect the Helicobacter pylori infection. In general, for the patients of group I the radioactivity of the samples of blown air showed a single maximum after ca 15-25 min. For patients of group II two maxima were obtained, the first after ca 5 min and the second after ca 15-25 min. Our investigations indicate that the 14C-urea breath test can not only be used to detect patients infected by Helicobacter pylori but also to establish the location (mouth, stomach) in the alimentary canal of the infection by that bacterium.