The most commonly detoxifying chelating agents in a decreasing order of softness are as follows: 2,3-dimercaptopropanol, D-penicillamine, calcium ethylenediaminetetraacetic acid. Chelating agents are used for detoxification therapy for removal of excessive levels of the metals from the target organs. The chelating agents compete with binding sites in the biological systems for complexing the metals producing binary or ternary water-soluble complexes. These complexes contain the metal, the therapeutic chelating agent, and another possible ligand. Metallothionein has been isolated from a wide range of tissues including liver, kidneys, and intestinal mucosa of a number of animals and several cell lines cultured on media containing Zn and Cd. The molecular weight of metallothionein is about 6500 and the metal content is about 7 to 10%; cysteine comprises 20 out of the 61 amino acid residues. The contaminating metal might enter the body through lungs, skin, or by ingestion.