Transition metal contamination poses a serious environmental and human health threat. The bioavailability of transition metals in environmental samples can best be assessed with living organisms. A transgenic strain of the free-living soil nematode Caenorhabditis elegans has been engineered for monitoring the bioavailability of metals. A reporter transgene consisting of a fragment of the promoter from the C. elegans metallothionein-2 gene (mtl-2
) that controls the transcription of a beta-galactosidase reporter (lacZ) has been integrated into the genome of this organism. By using these transgenic C. elegans, the toxicological response to metals in samples can be quickly measured with a simple histochemical staining assay. The C. elegans that contain the mtl-2
:lacZ transgene provide a more sensitive assay of exposure to cadmium, mercury, zinc, and nickel than 24-h LC50 assays or those using nematodes with heat-shock protein-based reporter transgenes. This study demonstrates that C. elegans that contain mtl-2
:lacZ transgenes can function as sensitive toxicological indicators of metals.