Partial deficiencies in enzymes activity of the heme biosynthesis pathway have been demonstrated in cultured skin fibroblasts and other tissues from patients suffering from congenital erythropoietic porphyria and hereditary coproporphyria. Using a new fluorimetric method, we have assessed quantitatively porphyrin biosynthesis from added delta-aminolevulinic acid in cultured fibroblasts of two congenital erythropoietic porphyria patients and one homozygous case of hereditary corproporphyria. The results were compared with those of the patients' parents and those of normal controls. All the porphyrins synthesized remained within the cells of normal subjects and of patients with congenital erythropoietic porphyria; these porphyrins were mostly (95%) protoporphyrin. The fibroblasts of the patient with homozygous hereditary coproporphyria synthesized the same amount of porphyrins, but only 25% were found within the cells, whereas 75% were found in the medium. The porphyrins found within the cells were coproporphyrin (25%) and protoporphyrin (75%); in the medium, only coproporphyrin was identified.