PDT relies on the combined action of photosensitizers localized on tumors and activated under laser light. Tumor cells response to PDT is determined by both the photosensitizer and the photoirradiation dose. However, the processes governing the resultant tumour destruction are still unknown and recent advances in the field of cellular biology have stimulated the research interest in this field. In this paper, the genotoxicity and chromosome damage on MIA pancreatic cancer cells, induced by red light and zinc tetrasulfonated phthalocyanines (ZnPcS4) was studied. The results showed that cells treated with a low concentration (1 (mu) M) of ZnPcS4 and a light dose of 6 J/cm2 indicated more than 70% lethality, 72 hrs after irradiation. The SCE assay, showed that treated cells with various light and drug doses, presented no genotoxic potential, as far as SCE levels were not different from those of the untreated (control) cells. Chromosomal analysis after optimum PDT treatment in various time intervals post irradiation, showed that there was not any significant chromosomal damage on cells, compared to the untreated (control) cells.