Increase of ornithine decarboxylase (ODC) activity is known to be associated with cell proliferation and, very likely, with tumour promotion. This prompted us to study the activity of ODC in gastric mucosa of patients with chronic atrophic gastritis that has been considered as a precursor of stomach cancer. Examination of 124 patients with this disease revealed the considerable increase in ODC activity in atrophic mucosa (29.8 +/- 2.9 vs 7.9 +/- 1.8 units in normal mucosa, p = 0.001). Supplementation of the patient's diet with beta-carotene (20 mg daily during 3 weeks) results in a statistically significant decrease in ODC activity in gastric mucosa. The data obtained confirm the possibility of application of ODC determination to the detection of early premalignant lesions and suggest the antipromoter activity of beta-carotene in gastric carcinogenesis.