the populations of seed-feeding birds (32). High levels of methylmercury have also been detected in fish from the Great Lakes region of North America. Mercury and organomercurial compounds are highly toxic. Methylmercury is 100 times more toxic than inorganic mercury and has been found to be mutagenic under experimental conditions (26). The solubility of inorganic and organic mercury compounds in lipids as well as their binding to sulfhydryl groups of proteins in membranes and enzymes (4) account for their cytotoxicity. Sources of mercury in the environment are both natural and anthropogenic in origin. In nature, cinnabar (red HgS) and metacinnabar (black HgS) are the most important mercury-containing ores. Livingstonite (HgSb4S7) and mercury-containing sulfide minerals such as tetrahydrite (6Cu2S * Sb2S3) are also considered important sources (18).