In extension of two previous series of experiments, the results of which have been partially reported (1), further long-term tests were performed with daily oral doses of 5 mg Pb++ in 2 subjects. Lead in whole blood, free protoporphyrin 9 (III) and 5-aminolevulinate dehydratase activity in erythrocytes, hemoglobin, and urinary 5-aminolevulinic acid and coproporphyrin III excretion values were measured. From experiences with 8 human experiments, three compartmental phases of early detectable lead effect were postulated: 1. an "erythrocytic" reaction of 5-aminolevulinate dehydratase activity, 2. "hepatic" reactions of 5-aminolevulinic acid and coproporphyrin, and 3. an "erythropoietic" reaction of protoporphyrin.