1. In 5 patients with erythropoietic (congenital) porphyria, unstained bone marrow preparations were studied and photographed by fluorescence and absorption microscopy. The same marrow slides were also studied after ordinary staining.

2. Two morphologically different varieties of normoblasts were observed, which were designated as normal and abnormal cells. Normoblasts of the abnormal variety exhibited nuclear inclusion bodies containing hemoglobin.

3. Red fluorescence, indicative of porphyrin, was found only in normoblasts belonging to the abnormal cell variety. Normal normoblasts failed to exhibit red fluorescence.

4. The red fluorescence originated predominantly from the normoblastic nucleus. The fluorescence intensity of the cytoplasm was usually very low. Polychromatophilic erythrocytes also exhibited only weak fluorescence.

5. These findings indicate the coexistence of two different lines of normoblasts. The nuclei of cells belonging to the abnormal line probably form excessive amounts of porphyrins and may release them into the plasma in the course of cellular maturation. It is believed that these cells carry the abnormal trait representing the "inborn error of metabolism."

6. A critical review of all cases of porphyria, reported as "congenital," has shown that on the basis of the presented data, it was possible to establish the diagnosis of erythropoietic (congenital) porphyria beyond reasonable doubt in but 34 instances. Nineteen of the patients were female, fourteen were male. In a few families, more than one case was observed in the same generation, but in no instance was the disease recognized in subsequent generations.

7. Erythropoietic porphyria is a congenital disease, entirely distinct from hepatic photosensitive ("cutanea tarda") porphyria. No evidence has been found of a genetic mixture of these two forms of porphyria.

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