The study was concerned with evaluation of the effect of human gastric juice (samples obtained from 157 subjects) on in vitro dimethylnitrosamine nitrosation by sodium nitrite versus gastric mucosa pathology (gastritis, polyps, ulcer, cancer), gastric juice pH and nitrate ion level. Also, the influence of gastric juice glycosaminoglycans and glycoproteins levels as well as N-acetylneuraminic acid isolated from porcine blood was assessed. An increased nitrosating activity of gastric juice was observed in cases of gastritis and ulcer and in those with low nitrate ion activity. Glycosaminoglycans and glycoproteins were found to inhibit nitrosation. N-acetylneuraminic acid in the concentrations used exerted no effect on dimethylnitrosamine nitrosation by sodium nitrite.