The addition of photoactivatable compounds to blood products has been suggested as a method for the inactivation of blood-borne viruses. We investigated the antiviral activity of one such compound, the lipophilic dye merocyanine 540 MC 540), with bacteriophages serving as surrogates for animal viruses. Four bacteriophages, phi x174, T7, PRD1 and phi 6, were treated with MC 540 (15 ug/mL) and filtered radiation (450-600nm) from a 750W projector with an intensity of 16-100 W/m2. Two of the phages, PRD1 and phi 6 have lipid components, and only phi 6 has an external lipid envelope. The survival curves of the different bacterial viruses clearly demonstrated different levels of sensitivity to photoinactivation by MC 540, with phi 6 being the most sensitive (D=1.5 kJ/m2) followed by T7 (21 times less sensitive). PRD1 with internal lipid was not photoinactivated by MC 540. For comparison, we also treated Herpes simplex virus with MC 540 and light and found it to be 28 times for sensitive than phi 6 with the same concentration of MC 540. Bacteriophage phi 6 may be suitable as a surrogate for enveloped viruses to study photoinactivation by lipophilic dyes, however, the results may be only useful qualitatively. This work was published in Photochemistry and Photobiology 1991;54:489- 493. Results of this study were discussed at an invited presentation at the American Sociology for Photobiology meeting in June, 1991 in San Antonio, TX. A manuscript will be published in Blood Cells.