Introduction The ancient Greeks and Romans first pointed out the deleterious effects of lead on the human body and the intervening centuries have only served to confirm their observations. The uses of lead which led to the contamination of man's environment have been varied@ but modern controls have resulted in a decline in lead exposure in both the home and industry. The most difficult problem confronting the toxicologist is a semantic one where the definition of lead intoxication becomes highly distorted because people feel that@ since lead is a toxic element@ any exposure@ no matter how minute@ will result in extensive damage to cells and tissues. Of course this is not true because the body's defense mechanisms are such that the average individual is in lead balance throughout his life-span. Therefore@ it is essential that we define and illustrate exactly what is meant by acute and chronic lead intoxication as well as what is meant by environmental lead contamination which reaches the individual through the food chain and in the air he breathes. It is also essential that we examine the biochemical mechanisms which may be involved in various aspects of lead exposure@ and point out their possible usefulness in the clinical evaluation of such exposures. Consideration will be given only to human exposures to lead because this is where out main interest is focused. However@ the value of animal studies in specific problems is recognized.