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INTRODUCTION The element mercury (Hg) and its compounds have no known normal metabolic function. Their presence in the cells of living organisms represents contamination from natural and anthropogenic sources; all such contamination must be regarded as undesirable and potentially hazardous (NAS 1978). The most important ore of mercury, cinnabar (mercuric sulfide), has been mined continuously since 415 BC (Clarkson and Marsh 1982). In the period before the industrial revolution, Hg was used extensively in gold extraction and in the manufacture of felt hats and mirrors; in the 1800's, it was used in the chloralkali industry, in the manufacture of electrical instruments, and as a medical antiseptic; and since 1900, it has been used in pharmaceuticals, in agricultural fungicides, in the pulp and paper industry as a slimicide, and in the production of plastics (Clarkson and Marsh 1982). Current world use of mercury is estimated at 10,000 to 15,000 metric tons annually (Boudou and Ribeyre 1983), of which the United States accounts for about 18% (Clarkson and Marsh 1982). The first cases of fatal mercury poisoning were reported for two men in a European chemical laboratory in 1865 (Das et al. 1982). The first documented human poisoning from an agricultural exposure to methylmercury occurred in 1940 (Das et al. 1982). As summarized by Elhassani (1983), exposure of humans to mercury compounds may result from dermal application (e.g., 1,600 infants in Argentina showed symptoms of Hg poisoning after a laundry treated their diapers with a Hg disinfectant), from diet (i.e., ingestion of Hg-contaminated fish, pork, seafoods, or grains), and from contact by respiratory routes (e.g., occupational exposure of mercury fungicide applicators in Nicaragua). Sporadic incidences of human poisonings have occurred in the United States, the Soviet Union, and Canada; and major epidemics have been reported in Japan, Pakistan, Guatemala, Ghana, Yugoslavia, and Iraq (Clarkson and Marsh 1982; Das et al. 1982; Elhassani 1983; Green and Kochen 1983). In 1972, for example, there were 6,530 hospital admissions within 18 months (459 hospital deaths) among Iraqi farmers who ate bread made from seed wheat treated with a methylmercury fungicide. A water soluble red dye was washed off the wheat, with the assumption that the mercury would be equally soluble. Before the wheat was consumed by humans, it was fed (without apparent effect) to chickens and other livestock for only a few days; it was not realized that a lengthy latency period was involved (Das et al. 1982; Elhassani 1983). There is no 1