Measurement of 6 -aminolevulinic acid synthetase (ALA-S) activity in human plasma has been carried out with samples from normal individuals, cases of erythropoietic porphyria (EP) and erythropoietic protoporphyria (EPP), and of the three principal forms of hepatic porphyria-acute intermittent porphyria, variegate porphyria, and porphyria cutanea tarda. The method of measurement depends on forma...