The localization of a hematoporphyrin derivative and its detection by fluorescence have been studied. It has been demonstrated that the derivative is a better localizer than the crude hematoporphyrin and that it has a greater tendency to accumulate in malignant than in nonmalignant tissue. In general, the greatest fluorescence is exhibited at the periphery of the tumor. By utilizing the acetic acid-sulfuric acid derivative of hematoporphyrin and a proper activating and viewing system, one can detect malignancy by fluorescence. This improved technique appears to be sufficiently more flexible than that used by earlier workers to warrant trial in human beings under certain circumstances.