Formerly known as the toxic substances list, the registry of toxic effects of chemical substances (RTECS) is a compendium of toxicity data abstracted from the scientific literature by the national institute for occupational safety and health (NIOSH) in compliance with the 1970 occupational safety and health act. The 1978 registry contains 124,247 entries: 33,929 are names of different chemicals with their associated toxicity data, and 90,318 are synonyms. For each compound, the following data are provided: prime name and synonyms; cas number; molecular weight and formula; irritation and toxic dose data, and references from which the data were abstracted; aquatic toxicity ratings; and citations to toxicology reviews, government standards and regulations, the NIOSH criteria document program, and the NCI carcinogenesis testing program. The registry provides information on known toxic and biologic effects of chemical substances for use by employers, employees, physicians, industrial hygienists, toxicologists, researchers, and others concerned with safe handling of chemicals. The absence of a substance from the registry does not indicate that the substance is not toxic. The toxic doses presented are not to be considered definitive doses for describing safe versus toxic doses for human exposure.