Virtually all protoporphyrin in erythrocytes of patients vith erythropoietic protoporphyria is bound to hemoglobin. The maximum of the fluorescence excitation spectrum of this protoporpliy-rin-hemoglobin complex shifted, with increasing concentration, from 405 nm to 389 nm. A similar shirt was observed wlien titrating a solution of free protoporphyrin with hemoglobin. The Soret maximum of free protoporphyrin itself. on the other hand, was not concentration-dependent. These observations indicate that spectrofluorometric measurements do not allow conclusions coticerning the mode of protoporphyrin binding to hemoglobin.Experiments on protoporphyrin exchange between the hemoglobins A, F and S reinforced the previously drawn conclusion that protoporphyrin is bound to hemoglobin at the heme-binding sites.