A novel test system for the detection of mutagenic and recombinogenic activity of chemicals is described in detail. Drosophila melanogaster larvae trans-heterozygous for the mutations multiple wing hairs (mwh) and flare (flr) are exposed to the test compounds for various periods of time ranging from 96 hr to 1 hr. Induced mutations are detected as single mosaic spots on the wing blade of surviving adults that show either the multiple wing hairs or flare phenotype. Induced recombination leads to mwh and flr twin spots and also to a certain extent, to mwh single spots. Recording of the frequency and the size of the different spots allows for a quantitative determination of the mutagenic and recombinogenic effects. This and earlier studies with a small set of well-known mutagens indicate that the test detects monofunctional and polyfunctional alkylating agents (ethyl methanesulfonate, diepoxybutane, mitomycin C, Trenimon), mutagens forming large adducts (aflatoxin B1), DNA breaking agents (bleomycin), intercalating agents (5-aminoacridine, ICR-170), spindle poisons (vinblastine), and antimetabolites (methotrexate). In addition, the test detects mutagens unstable in aqueous solution (beta-propiolactone), gaseous mutagens (1,2-dibromoethane), as well as promutagens needing various pathways of metabolic activation (aflatoxin B1, diethylnitrosamine, dimethylnitrosamine, mitomycin C, and procarbazine). The rapidity and ease of performance as well as the low costs of the test necessitate a high priority for validation of this promising Drosophila short-term test.