The present study investigates the photochemical properties of a potential photosensitiser, indocyanine green (ICG), in an in vitro HeLa cell system. Cell proliferation was studied after a combined effect of ICG, at a concentration range of 24-94 microM, and therapeutic laser irradiation at several different energy densities. In addition, ICG cytotoxicity was evaluated in HeLa cells and V79 Chinese hamster by the MTT assay. Phototoxicity was evaluated at 1, 24, and 48 h after irradiation. No phototoxic effect was detected 1h after irradiation. The maximum phototoxic effect of ICG on HeLa cells was detected for an ICG concentration of 94 microM, a laser output of 360 mW, and an energy density of 99 J/cm(2) at 24h after irradiation. Potentiation of the ICG phototoxic effect was achieved by adding 20 microM H(2)O(2), which was a non-toxic concentration for HeLa cells in this experimental design. At 48 h after laser irradiation a statistically significant difference was found between the toxicity of ICG plus peroxide, as compared to ICG alone. The addition of H(2)O(2) at a concentration of 20 microM caused a significant increase in phototoxicity of ICG for HeLa cells. Our results confirm that ICG could be a perspective agent for use in photodynamic therapy and that its phototoxic effect can be potentiated by addition of an oxidative agent.