We investigated the efficiency and the mechanism of action of two glycoconjugated tetraphenyl porphyrins in their photoreaction with T7 bacteriophage. Both types of porphyrins sensitized the photoinactivation of T7, but the slopes of inactivation kinetics were markedly different. Our result suggests that both type I and type II reaction play a role in the virus inactivation. Optical melting studies revealed structural changes in the protein part but not in the DNA of the photo-chemically treated nucleoprotein complex. Polymerase chain reaction (PCR) analysis failed to demonstrate any DNA damage.