1. Erythrocytes from patients with erythropoietic protoporphyria (E.P.P.) were incubated with radioactive iron, washed and centrifuged. Serial layers were removed from the packed cell columns. The radioactivity and protoporphyrin content of each layer was measured. Blood and marrow aspirates were examined by fluorescence microscopy. 2. The youngest erythrocytes were most fluorescent and contained most protoporphyrin. The amounts present in the cells of different layers progressively decreased from top to bottom of the packed cell column. It is concluded that the protoporphyrin content decreases during the life of the erythrocytes, probably by a process of elution. 3. Although little 59Fe entering the erythrocytes is incorporated into haem, a relationship was observed between iron uptake and protoporphyrin content, which may not be entirely due to aging. This may reflect a mutual dependence of both variables on haem feed-back control. 4. Fluorescence of normoblasts was not detected, indicating that the chief accumulation of porphyrin occurs soon after loss of the nucleus. A hypothesis is suggested to explain these findings.