A new technique for the intracellular localization of minute amounts of heme and hemoproteins is described. The specimen is treated with 1.5 M perchloric acid in the presence of SH groups, followed by ultraviolet light irradiation in a fluorescence microscope. This fixes the proteins in situ and converts the heme to a porphyrin which fluoresces and is readily visualized. With this technique, hemoglobin has been demonstrated in the nuclei of avian erythrocytes, and in the nuclei of human normoblasts at an earlier stage than previously described. In addition, hemoproteins, presumably cytochromes, have been detected in the cytoplasm and nuclei of myelocytes, in thymus lymphocyte nuclei, in chick embryo liver cytoplasm, and in chick embryo somites.

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