To provide further background data for the somatic mutation and/or recombination tests in Drosophila melanogaster, we have evaluated the response in 3 assays (zeste-white, white-ivory and wing spot) of 5 chemicals classified by the U.S. National Toxicology Program (NTP) as genotoxic non-carcinogens (or ambiguous). The selected compounds were 2-chloromethylpyridine, 1-nitronaphthalene, 4-nitro-o-phenylenediamine, 3-nitropropionic acid and p-phenylenediamine. Our results show that all the compounds tested produce significant increases in the frequency of mutant clones, in at least one of the assays, p-phenylenediamine being the compound which presents a clearer mutagenic activity, and the wing spot test, the assay that detects more genotoxic compounds (4/5).