Today, health professionals are concerned about whether asymptomatic or mildly symptomatic increases in lead absorption in preschool children is one of the important factors causing minimal brain dysfunction, which only becomes evident later during the school years. This concern arises, in part, because current screening data show that 5 to 10% of the children tested recently in the United States have a degree of increase in lead absorption sufficient to cause metabolic derangement in heme synthesis, but insufficient, with rare exception, to cause classical acute clinical symptoms of plumbism. These screening data are disturbing because similar increases in lead absorption in suckling (but not older) experimental animals have been shown in some studies to be followed by the delayed appearance of subtle deficits in learning ability and aberrations in behavior. Derangement of hemoglobin synthesis is the first adverse effect of increased lead absorption now detectable. Intervention on the basis of the first or critical effect of a toxic agent before more serious effects occur has advantages from the viewpoint of preventive medicine. Current CDC guidelines for prevention of childhood lead poisoning are based on this concept and recommend the use of micro-scale erythrocyte protoporphyrin tests in conjunction with micro blood lead tests for early detection of children with disturbed heme synthesis caused by lead.