The role of sulphur and copper-inducible thiol-rich protein in the copper tolerance- of the grass Deschampsia cespitosa (L.) Beauv. was investigated. Sulphur deficiency, induced by growing tillers in nutrient solution without sulphur had no effect on the copper tolerance of two clones from a site contaminated with copper and nickel. In contrast sulphur deficiency increased the susceptibility of two non-tolerant clones from an uncontaminated site-to copper toxicity. Tolerance of a copper-tolerant clone screened from seed of a non-tolerant population was unaffected by sulphur deficiency. Sulphur deficiency reduced the amount of copper-inducible thiol-rich protein in roots of both the tolerant and non-tolerant clones exposed to excess copper. Roots of the tolerant clones grown in micronutrient copper solutions without sulphur for 26 d and then exposed to 1.61 μM copper for a further 4 d both produced 75% less copper-inducible thiol-rich protein than those grown with sulphur. In an experiment with one of the tolerant clones, the amount of thiol-rich protein did not increase between 4 and 30 d of exposure to excess copper despite a 43% increase in copper in the cell-free extract. Sulphur deficiency reduced the amount of thiol-rich protein to a lesser extent in the non-tolerant clones, with a 57% decrease in one clone and a 31% decrease in the other. In the presence of sulphur both non-tolerant clones produced considerably more thiol-rich protein when exposed to excess copper than did the tolerant clones.