Earthworms have been studied as a readily available, easily maintainable and cheap test species to develop toxicity test systems as an alternative to in vivo rodent bioassays. The results obtained, mainly with metals, since the programme was initiated about 15 years ago, are summarized in this review. Metals proved more toxic to rodents than earthworms; the relative order of toxicity for different metals was similar in both species. The effects of metals and other compounds on enzyme activity [glutathione S-transferase (GST) and catalase] and the induction of metallothioneins in earthworms are described. It is concluded that useful preliminary information can be obtained in earthworms on the acute toxicity of inorganic metal ions and on the relative order of toxicity of groups of metal salts.