Photodynamic therapy (PDT) is a photochemotherapeutic process that is used for the treatment of cancer. Photofrin is the most widely used photosensitizer, however, the chemical composition of Photofrin is unclear and it has a low absorption in the therapeutic wavelength (600-900 nm). This factor has stimulated research in synthesis and testing of new photosensitizers. This in vitro study evaluated the effectiveness of a Zn sulfophthalocyanine (ZnPcS(mix)) as a potential photosensitizer in the treatment of human lung cancer. Lung cancer cells (A549) were divided into four groups: group 1 was control cells receiving neither light nor drug; group 2 was light control for cells exposed to laser irradiation at a fluence of 4.98 J/cm(2); group 3 was drug control for cells incubated with 15.8 μM photosensitizer and not exposed to laser irradiation, while group 4 was cells receiving the experimental treatment with 15.8 μM photosensitizer and irradiation with 4.98 J/cm(2). Laser irradiations were performed using a 636-nm diode laser with an output power of 110 mW at 4.98 J/cm(2). Changes in cellular responses were evaluated by cell morphology, viability, proliferation, and cytotoxicity. While control groups 1, 2, and 3 showed no changes in cell morphology, viability, proliferation, or cytotoxicity, group 4 receiving both photosensitizer and irradiation showed changes in cell morphology, a decrease in cell viability and proliferation, and an increase in cytotoxicity, cell death, and cell membrane damage. Irradiation or photosensitizer alone had no effect on the lung cancer cells since the cells remained viable and showed no evidence of damage. However, irradiation in the presence of a photosensitizer induced cell death.