This invention provides novel compounds for therapy of tumors susceptible to necrosis when an appropriate compound is administered followed by illumination with light of a wavelength absorbed by the compound, the compounds being dihydro porphyrins of the formula, or the corresponding tetra-hydro prophyrins: ##STR1## where n=1 to 3 and each substitutent R, the same or different and at the same or different position in its respective substituent phenyl ring, is an hydroxy group.