Ultracentrifugational studies with deoxyribonucleic acid (DNA) extracted from phage exposed to light in the presence of either 0.25 or 8.5 μg of proflavine per ml reveal that the lethal damage of R×1 renders DNA alkali-labile, with lethality resulting from damage that occurs singly on either strand or simultaneously on both strands. Apparently nonlethal damages temporally associated with R×1 consist of (i) heat and alkali-labile cross-links (which produce undenatured DNA that migrates at 45S) and (ii) heat-labile bonds. The formation of 45S material is a linear function of light dose, and the production of this material ceases when R×2 appears at the higher dye concentration. No tendency to plateau is seen at 0.25 μg of dye per ml. The nature of the lethal damage of R×2 could not be determined. Damages that were temporally associated with R×2 at 8.5 μg of dye per ml were heat-labile, alkali-stable cross-links (undenatured DNA, 38 to 41S; alkali-denatured-reneutralized DNA, 85 to 89S) and some double-stranded breaks. No such changes were seen at 0.25 μg of dye per ml.