Photodynamic therapy (PDT) is an increasingly used treatment for various types of cancer. The principle of PDT involves the administration of a photosensitizer, followed by a distribution interval, and subsequent illumination of tumour area with light of an appropriate wavelength to excite the sensitizer to its triplet state. The aim of the study was to determine the level of lipid peroxidation and the level of thiol groups (-SH) in A549 cells after PDT. The final product of fatty acid peroxidation--malondialdehyde--was quantified Spectrophotometrically, based on a set of MDA standards of known concentration. Protein damage was based on Ellman's method. The level of lipid peroxidation was significantly higher for cells after PDT, comparing to control cells. We observed much lower concentrations of -SH groups for cells after PDT treatment, in comparison with respective values in control cells. In conclusion, PDT with Ph II induces lipid peroxidation with accompanying protein damage in A549 cells, what can lead to distinct epidemiological, pathological and clinical features.