Mutants in the gene flu-2 of the free-living nematode Caenorhabditis elegans are characterised by an altered autofluorescence of the intestine cells, from the light blue of wild-type to a dull green colour. The properties of flu-2 mutants have been investigated. L-kynureninase activity has been detected in wild-type C. elegans. The flu-2 mutants have markedly reduced kynureninase activity, as predicted earlier from chromatographic analysis of tryptophan catabolites of wild-type and mutant worms. Associated with this enzymatic block, all flu-2 mutants have enhanced sensitivity to ethyl methane sulfonate (EMS) and gamma-rays.