XANTHINE dehydrogenase from Drosophila melanogaster is of particular interest since mutants at two separate loci lack this enzyme1 while a mutant at a third locus produces about 25 per cent of normal activity2. In an effort to detect strains of flies with genetically altered forms of xanthine dehydrogenase we devised a technique involving paper-strip electrophoresis in which this enzyme is located after electrophoresis by spraying the strip with the enzyme's substrates, incubating it at 25° C, and then using paper chromatography to separate the fluorescent product from the substrate which also fluoresces. This technique is very simple, and has potential application to other enzymes. Paper electrophoresis of xanthine dehydrogenase in rat serum has previously been reported using the technique of elution and direct assay3,4.