Abstract Subcellular fractions from Drosophila melanogaster, known to have several xenobiotic-metabolizing enzymatic activities, were investigated with respect to their ability to biotransform compounds that require metabolic activation before exerting mutagenic effects. Nitrofurazone, dimethylnitrosamine, cyclophosphamide and 2-acetylaminofluorene were activated to mutagens upon incubation with Drosophila microsomes or 20 000 × g supernatant: mutagenicity was observed in Chinese hamster ovary cells, Escherichia coli strains 343/113/R-9 and 343/113/ uvrB, and Salmonella typhimurium TA1538. Under the conditions used, microsomal preparations of Drosophila were not able to activate benzo[ a]pyrene to a mutagen for Salmonella typhimurium TA98. The spectrum of mutagenic effects observed shows some correlation with the known mutagenicity of these compounds in vivo in Drosophila melanogaster. Drosophila microsomes appeared to be at least as active as rat-liver microsomes when compared in this type of mutagenicity testing.