This volume evaluates carcinogenicity to humans of 19 chemicals that are carcinogenic to the thyroid follicular-cell epithelium in rodents. These included some so-called "anti-thyroid" drugs (methimazole, methylthouracil, propylthiou-racil and thiouracil); some sedatives (doxylamine succinate and phenobarbital); and some other drugs including the systematic antifungal antibiotic griseofulvin, the diuretic spironolactone and the antibacterial sulfa drugs sulfamethazine and sulfamethoxazole. Other chemicals are or have been used in agriculture as pesticides (amitrole, chlordane/heptavhlor, hexachlorobenzene and toxaphene), in foods and cosmetics (kojic acid), in hair dyes (2,4-diaminoanisole) or as industrial chemicals (N,N'-diethythiourea, ethylenethiourea and thiourea). Five of the 19 (N,N'-diethythiourea, doxylamine succinate, kojic acid, methimazole and sulfamethazine) were evaluated for the first time. This series of evaluations specifically included agents for which mechanisms of carcinogenesis may operate in rodents that do not operate in humans, at least under conditions of realistic human exposure. Evidence from epidemiological studies and from toxicological studies in experimental animals provide compelling evidence that rodents are substantially more sensitive than humans to the development of thyroid tumours in response to thyroid hormone imbalance. From the available epidemiology studies, there was no indication of excess thyroid cancer risk in humans exposed to any of these agents Amitrole, ethylenethiourea and sulfamethazine - all with sufficient evidence or carcinogenicity in experimental animals - were placed into group 3 (not classifiable as to carcinogenicity to humans), because the carcinogenic effects observed in animals would not be expected to occur in humans. Amitrole, ethylenethiourea and sulfamethazine produce thyroid tumours in mice and rats by a non-genotoxic mechanism, which involves interference with the functioning of thyroid peroxidase. This results in reduction of circulating thyroid hormone concentrations and increased secretion of thyroid-stimulating hormone, which is not expected to occur in humans under realistic conditions of exposure. Chlordane/heptachlor, griseofulvin, hexachlorobenzene, methylthiouracil, phenobarbital, propylathioracil, thiouracil and toxaphene were evaluated as possible carcinogenic to humans (group 2B) on the basis of sufficient evidence for carcinogenicity to exeprimental animals. 2,4-Diaminoanisole was classified as possibly carcinogenic to humans (group 2B) on the basis of sufficient evidence of carcinogenicity in animals at multiple organ sites including thyroid, together with evidence that the compound acts by a genotoxic (DNA-reactive) mechanism.