Activation of the complement system has emerged as a critical event in the response of tumors to photodynamic therapy (PDT) due to its involvement in the vascular effects as well as in the inflammatory and immune reactions observed with this treatment modality. However, the exact mechanism of PDT-induced complement activation has not been precisely characterized. The present study examines the potential of PDT at the cellular level to directly activate the complement system. Mouse tumor SCCVII cells treated by Photofrin-based PDT and post-incubated in the presence of homologous (mouse) serum were analyzed by flow cytometry for binding of complement proteins on their surface. The results show that PDT induced the fixation of complement C3 protein, probably in the form of its activated fragments, and of the terminal membrane attacks complex of complement on the treated SCCVII cells. Deposition of C3/C3 fragments on human umbilical vein endothelial cells (HUVEC) treated by Photofrin-PDT and post-incubated in the presence of human serum was also detected. Complement fixation was preferentially elevated on SCCVII cell undergoing PDT-induced apoptosis. The induction of surface expression of heat shock protein 70 (HSP70) on PDT-treated cells also triggered complement deposition, since the presence of anti-HSP70 antibodies during the post-PDT incubation blocked the anchoring of C3/C3 fragments to SCCVII cells. The fact that PDT-treated cells are recognized by the complement system as their target adds an important element for understanding the mechanism of tumor response to PDT.