We are investigating the interrelationships between levels of total N-nitroso compounds (NOC), genotoxic activity (both before and after nitrosation), degree of bacterial colonization in gastric juice and degree of severity or absence of precancerous lesions of the stomach. The mean level of constitutive total NOC in gastric juice was similar in the different groups of patients, but it was higher in acidic gastric juice (n = 30) than in gastric juice at pH greater than 4.5 (n = 12). Acid-catalysed nitrosation of gastric juice in vitro increased the concentration of total NOC by up to several thousand fold, to a maximum of 1330 mumol/l. Genotoxicity, expressed as SOS-inducing potency per 100 microliters of gastric juice was measurable in only 20% of gastric juice samples tested. After acid-catalysed nitrosation, however, all samples showed genotoxic activity, the mean SOS-inducing potency being four to seven times greater than the corresponding constitutive value. There was no association between the mean SOS-inducing potency of gastric juice and the severity of precancerous lesions. The mean SOS-inducing potency of neutral or basic gastric juice was slightly greater than that of acidic samples. In a kinetic study on N-nitrosation of gastric juice in vitro, a mixture of amino and amido substrates was nitrosated; both qualitative and quantitative individual differences in nitrosatable substrates in gastric juice were seen. Fractionation of acidic, neutral and basic nitrosated gastric juice samples revealed a preponderance of nonvolatile, unknown NOC with varying polarities. The results of our study suggest that only pH determines the nature and level of precursors of NOC and of nitrosation-dependent genotoxins in gastric juice.