We have shown that membrane attack complex (MAC) formation via the activation of the alternative pathway plays a central role in the laser-induced choroidal neovascularization (CNV). This study was undertaken to understand the role of a complement regulatory protein, CD59, which controls MAC assembly and function, in this model. CNV was induced by laser photocoagulation in C57BL/6 and Cd59a(-/-) mice using an argon laser. Animals from each group were sacrificed on day 1, 3, 5, and 7 postlaser. Retinal pigment epithelium-choroid-scleral tissue was examined to determine the incidence and size of CNV complex, and semiquantitative RT-PCR and Western blot analysis for CD59a was studied. Recombinant soluble mouse CD59a-IgG2a fusion (rsCD59a-Fc) protein was injected via i.p. or intravitreal routes 24 h before laser. Our results demonstrated that CD59a (both mRNA and protein) was down-regulated during laser-induced CNV. Cd59a(-/-) mice developed CNV complex early in the disease process. Increased MAC deposition was also observed in these Cd59a(-/-) mice. Administration of rsCD59a-Fc inhibited the development of CNV complex in the mouse model by blocking MAC formation and also inhibited expression of angiogenic growth factors. These data provide strong evidence that CD59a plays a crucial role in regulating complement activation and MAC formation essential for the release of growth factors that drive the development of laser-induced CNV in mice. Thus, our results suggest that the inhibition of complement by soluble CD59 may provide a novel therapeutic alternative to current treatment.