The common compost earthworm Eisenia fetida was used for toxicity testing ofmercury, in a well-characterized agricultural soil obtained from a local field (Ås,Norway). Groups of five or ten clitellate earthworms were exposed to variousconcentrations of mercury in soil, added in the form of mercuric chloride (HgCl2) indistilled water. The effects on survival, burrowing behaviour, reproduction, growth,and glutathione levels were recorded. Very low mercury concentration (0.22 mg/kgsoil) significantly stimulated cocoon production after two weeks, but higherconcentrations reduced it after four weeks (EC50 for reproduction was approximately7.4 mg Hg/kg soil). Protein and glutathione (non-protein thiols) were analyzed bystandard spectrophotometric methods. Earthworms exposed to mercury (22 to 740 mgHg/kg soil for four weeks) experienced a threefold increase in glutathione levels frombackground levels of approximately 0.62 µmol/g earthworm wet weight. Pre-exposureto mercury (22 mg/kg soil for one week) strongly increased survival in a subsequentstandard test carried out one week after the pre-treatment. Differences in cocoonproduction and growth between pre-exposed and non pre-exposed worms weresmaller, though statistically significant. Glutathione levels were similar in preexposedand non pre-exposed earthworms, suggesting that increased glutathionelevels did not cause the increased survival in pre-exposed animals. Internalconcentrations of mercury were determined in both pre-exposed and non pre-exposedearthworms, and followed a one site binding hyperbola. This reached a maximumaround 740 mg Hg/kg soil, corresponding to the internal concentrations 139 + 9 and165 + 16 µg Hg/g earthworm wet weight in two independent experiments. The shapeof the accumulation curves resembled the shape of the curves for glutathioneconcentrations plotted against mercury concentrations in soil, suggesting thatglutathione levels in earthworms are closely related to internal concentrations ofmercury. Therefore, glutathione levels may be a useful biomarker for mercuryexposure.