The action spectrum of disulfonated aluminum phthalocyanine (AIS2Pc) as a photosensltizer for photodynamic therapy (PDT) was evaluated In vivo from 640 to 710 nm on two murine tumors with different biological properties. Mice bearing MS-2 fibrosarcoma and mice bearing the pigmented tumor B16 melanoma were injected with 5 mg/kg of AIS2Pc and irradiated with a light dose of 50 mW for 10 min for the MS-2 fibrosarcoma and of 100 mW for 10 min for the B16 melanoma. The action spectrum for both tumors presents a red shift with respect to the absorption spectrum of AIS2Pc in saline. The effectiveness is limited for short wavelengths (≤655 nm for MS-2 and ≤660 for B16), whereas it increases at longer wavelengths and reaches its maximum at a peak (672 nm). For wavelengths beyond 672 nm the photo-dynamic activity remains up to 710 nm despite the significant decrease in absorption. The results obtained for both murine tumors seem therefore to indicate that an appreciable modification of the absorption spectrum takes place when AIS2Pc is incorporated into tissues following systematic administration.