The lipophilic photosensitizer, merocyanine 540 (MC) has been proposed for use as an agent to reduce the transmission of virus by cellular blood components. In a previous study, we demonstrated the antiviral activity of MC 540 in the presence of visible light (450-600 nm). The inactivation of lipid-enveloped viruses by dye and light is most likely mediated by reactive singlet oxygen. Platelets are highly susceptible to photodamage by MC 540 and demonstrate marked morphological alterations, a disruption of the normal response to agonists, and a spontaneous release of granule contents. Significant photodamage to the platelet membrane upon treatment with dye and light has been demonstrated in studies of MC 540 binding to membranes in the absence or presence of exogenous albumin, by measurement of 14C-arachidonic acid release from platelet membranes, and by SDS-polyacrylamide gel electrophoretic (SDS-PAGE) analyses of alterations of the migration of membrane proteins following treatment. In addition, MC 540 with and without light caused the generation of microvesicles (MV) from platelet membranes, indicating activation of platelets, even in the dark. Microvesicles have been isolated by high speed centrifugation, solubilized and analyzed by SDS-PAGE. In the dark, MC 540 caused an immediate shedding of protein-containing MV. The time-dependent, light-independent effects of MC by MC 540 and light produced microvesicles with a different protein pattern from that produced in the dark. The individual proteins incorporated into microvesicles are currently being identified by immunoblotting using antibodies to major platelet glycoproteins, cytoskeletal proteins, and granule proteins. Development of techniques to analyze microvesicles by flow cytometry is also underway. Results of this study were presented at the annual meeting of the American Society for Photobiology in June, 1991. A manuscript is in preparation.