Nitrated polycyclic aromatic hydrocarbons, including carcinogens, may be non-mutagenic in microorganisms because bacterial nitroreductases are unable to reduce their nitro function to proximate mutagenic hydroxylamines. The reduction of the nitro moiety can be accomplished chemically in situ using zinc dust. The procedure, which is compatible with the Salmonella mutagenicity assay, was used to generate mutagens from chemicals which otherwise are non-mutagenic even in the presence of microsomal preparations.