We studied the morphology of the A549 cell line (human lung carcinoma cells) before and after photodynamic therapy (PDT) by atomic force microscopy. PDT was induced by an efficient light-emitting diode source with total light dose of 15 J cm(-2) in the presence of the sensitizer zinc-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrine. In the presence of molecular oxygen, light activation of the photosensitizer, which accumulates in cancer cells, leads to the local production of reactive oxygen species (ROS). This is one of several reasons leading to cell death, and in some cases we could observe signs of apoptosis. We detected the kinetics of ROS production to be dependent on the presence of green tea extract.