Free histidine as well as histidine in the form of dipeptides present in food samples or biological material interacted with the Ames mutagenicity test. This event becomes especially important for test compounds that exert only weak 'mutagenic' effects. Histidine bound to large molecules and methyl derivatives did not affect the Ames test. For the avoidance of misinterpretations, it is therefore indispensable to determine or eliminate the interacting forms of histidine when protein-containing samples are tested in the Ames test. Extraction methods using sorbants and solvents that contain contaminants or can induce chemical reactions with biological test substances should be well controlled or avoided. Solvents such as alcohols, esters and aliphatic hydrocarbons that would minimally react with such test material should therefore be used for extraction.