Photodynamic therapy (PDT) is systemic administration of tumor localizing photosensitizers and subsequent Irradiation with light of the appropriate wavelength. The combination of drug uptake in malignant tissues and selective delivery of laser-generated light provldea effective therapy, with efficient tumor cytotoxicity and minimal normal tissue damage. There have been few studies of the effects of photoactlvated photosensitlzers on the host Immune response. Since immunity is Important in the control of tumor growth and spread, we have examined, in our laboratory, the effects of photoactlvated phthalocyanlnea on the antitumor immune response. Immunosuppressed and normal mice bearing the MS-2 fibrosarcoma trested with 5 mg/kg of aluminum dlsulfo-natad phthalocyanine (AlS2Pc) and then the tumor mass exposed to laser light (100 mW/cm2 x 10 min) or treated with surgical excision of the tumor survived Indefinitely, with no difference between the different groups. The survivors, tumor-free 100 daya after the treatment modalities described above, were rechallenged with the parental MS-2. Some groups of surviving animals were Immunosuppressed with cyclophosphamide before the Injection of the tumor. Roslatanco to rechallenge waa evidenced only In normal surviving animals cured by PDT, while the immunodepressed surviving animals and animals cured by surgery died of tumor. Finally, mice, cured by PDT and tumor-free, rechallenged with L1210 and P388 murine leukemias did not survive. These results suggest that a potential and specific 'antitumor Immunity' is induced by PDT with photoactivated AlS2Pc.