Abstract Sodium nitrite and two primary aromatic amines, viz. amino antipyrine (AAP) and aniline, were preincubated in vitro with human gastric juice. The resulting derivatives — presumably diazonium salts — were directly mutagenic in the Salmonella test. The mutagenic response was more pronounced in the case of AAP, while toxic effects narrowed the range of activity of the aniline derivative. These patterns are consistent with the findings of independent colorimetric analyses, showing that the AAP derivative is more stable at 37°C than the aniline derivative.