The photosensitizing efficiency of six dyes--proflavine, 9-aminoacridine, ethidium bromide, thiopyronine, pyronine and acridine red--have been compared on the basis of the inactivation of sensitized T4 phage caused by light irradiation. This reaction was only measurable after diffusion of the dye through the phage capsid and was not observed in the presence of either chloroquine or quinacrine; it followed a single-hit kinetics as a function of the irradiation time. With each dye, a double reciprocal plot of the inactivation constant versus the dye concentration present gave rise to a linear relationship. From this relation, parameters were deduced which expressed the relative photosensitizing efficiencies. Dye-binding to the phages was measured and the proflavine-mediated inactivation appeared to be related to the amount of strongly bound molecules. Such a conclusion could not be reached in the case of 9-aminoacridine and ethidium bromide, which were much less efficient photosensitizers than proflavine, but which were also strongly bound to the phages. Thiopyronine was weakly bound to the phages; it had, however, the highest photosensitizing activity observed. These results indicate that various mechanisms are involved when the phage photosensitization is due to one dye or another.