Metals differ from most synthetic organic chemicals in that their clinical manifestations are well known and methods for their measurement in the body are generally well established. Since metals are ubiquitous, special care should be taken to identify the source, whether dump site or not. Isotopic ratios may be used for lead. Time of exposure may be highly variable so estimates will be necessary of integrated "dose-commitment." Transmission to man will follow many pathways. The contamination of children's hands and clothing by dust may be an important route. Because effects are so different, the chemical species (e.g., organic versus inorganic forms) of each metal must be identified. Exposure assessment requires identification of suitable indicator media, usually blood in the case of lead, urine with cadmium and inorganic mercury, and blood or hair with regard to methylmercury. Human head hair may have considerable potential, as it may provide a recapitulation of past exposures. The first health complaints associated with most metals are usually nonspecific. The complex social, political, and legal issues strongly indicate the need for objective tests for health effects. Most important is the identification and measurement of the critical effect, i.e., an effect that alerts the public health authorities that further exposure should cease. For example, in the case of lead, the critical effect is hematologic; with cadmium it is the presence in urine of abnormally high concentration of small molecular weight protein; and with mercury no early objective test has yet been devised.