Micro-organisms commonly present in human saliva and three DSM strains (Helicobacter pylori, Campylobacter jejuni and Neisseria cinerea), which can be isolated from the human gastro-intestinal tract, were assayed in vitro for their capacity to catalyse N-nitrosation of a series of medicinal drugs and other compounds. Following incubation at pH 7.2 in the presence of nitrate (or nitrite) for up to 24 (48) h, the yield of N-nitroso compounds (NOC) was quantified by HPLC equipped with a post-column derivatization device, allowing the sensitive detection of acid-labile and acid-stable NOC. Eleven out of the 23 test compounds underwent bacteria-catalysed nitrosation by salivary bacteria, the yield of the respective nitrosation products varying 800-fold. 4-(Methylamino)antipyrine exhibited the highest rate of nitrosation, followed by dichlofenac > metamizole > piperazine > five other drugs, whilst L-proline and L-thioproline had the lowest nitrosation rate. Ten drugs including aminophenazone, cimetidine and nicotine, did not inhibit bacterial growth, allowing transitory nitrite to be formed, but no N-nitroso derivatives were detected. Three drugs inhibited the proliferation of bacteria and neither nitrite nor any NOC were formed. Using metamizole as an easily nitrosatable precursor, two strains, Campylobacter jejuni and Helicobacter pylori, were shown to catalyse nitrosation in the presence of nitrite at pH 7.2. As compared to Neisseria cinerea used as a nitrosation-proficient control strain, H. pylori was 30-100 times less effective, whilst C. jejuni had intermediary activity. The results of our sensitive nitrosation assay further confirm that bacteria isolated from human sources, possessing nitrate reductase and/or nitrosating enzymes such as cytochrome cd1-nitrite reductase (Calmels et al., Carcinogenesis, 17, 533-536, 1996), can contribute to intragastric nitrosamine formation in the anacidic stomach when nitrosatable precursors from exogenous and endogenous sources are present.