Much research has been done on the detoxification system involved when a plant absorbs a heavy metal, especially cadmium. For example, as to relevant ligands that combine with the cadmium absorbed by a plant in order to detoxify it, organic acids, such as citric acid, malic acid, etc., and amino acids, such as cysteine, histidine, glycine, etc., are well known. Furthermore, glutathione, phytochelatins, and metallothioneins, which contain cysteine in a high concentration, are known to contribute to detoxification of cadmium in plants. Up to now we have introduced a metallothionein-synthesizing gene (CUP1) and the gene of cysteine synthase into plants to determine what gives plants the ability to tolerate heavy metals. However, it is unknown which gene should be introduced to achieve the highest tolerance.This is because it is unknown which of the relevant ligands mentioned above contributes to the detoxification most greatly when a plant absorbs a heavy metal, for example, cadmium. In other words, even if it turns out that each ligand contributes to detoxification of cadmium, how each relevant ligand acts and to what extent in the detoxification of cadmium remains to be determined. So we examined which ligands contribute to the detoxification of cadmium-treated plants.Arabidopsis thaliana (cv. Columbia) was used for the experiments as a model plant. Ten days after sowing their seeds, A. thaliana were transplanted to a culture solution and cultivated for 14 days. Thereafter they were transplanted to a culture solution containing 0, 10 or 50然 CdCl2 and cultivated for 6 days. Consequently, the growth of A. thaliana treated with 10然 Cd was almost the same as that of the untreated plants. However, when 50然 Cd was present, the chlrorosis phenomenon appeared in the leaves, and the growth decreased remarkably. The content of cysteine in the shoot of this A. thaliana was increased by 10然 Cd. Moreover, serine and methionine contents were increased remarkably by 50然 Cd. Cd did not appear to influence the glutathione content in the shoot, whereas 50然 Cd increased it remarkably in the roots. On the other hand, such ligands of Cd are compounds which contain cysteine in high concentration. Then, the influence of Cd on the expression of genes involved in sulfur metabolism in plants was considered.Consequently, in the presence of 10然Cd, the expression of the phytochelatin synthase gene in the shoot increased remarkably; whereas that of genes for other sulfur-metabolizing enzymes did not change.When plants were treated with 50然 Cd, the expression of glutathione synthase gene and phytochelatin synthase genein both roots and shoots increased remarkably. However, the expression of the genes for other enzymes was not influenced. From these results, the grade of the contribution of thiol compounds to cadmium tolerance of plants was considered.