The pharmacokinetic behaviour and phototherapeutic effectiveness of 2,3-dihydro-5,15-di(3,5-dihydroxyphenyl)porphyrin (SIM01), a new diphenylchlorin photosensitizer incorporated into dimyristoyl phosphatidylcholine (DMPC) liposomes, were studied in vivo in nude mice bearing HT29 human adenocarcinoma. The photophysical and photochemical specificity of SIM01 are a strong absorption at 647 nm, high photosensitizing efficiency and a rapid pharmacokinetic profile making SIM01 an attractive candidate for PDT. The pharmacokinetics in vivo, as determined by fiber spectrofluorimetry, showed that tumor concentration was found maximal for SIM01 and SIM01-DMPC 12h after injection, with better uptake for the liposomal formulation. With a 2 mg kg(-1) dye dose, optimal PDT response occurred when the interval between injection and irradiation was 12h for both formulations, with laser irradiation of 300J cm(-2) (650 nm, 300 mW). At day 12 after treatment involving a 12-h interval between injection and irradiation, tumor growth was decreased by 26% for SIM01 (P=0.005) and 35% for SIM01-DMPC (P=0.001) as compared to the untreated group.SIM01 would appear to be a powerful sensitizer characterized by high in vivo phototoxicity and rapid tissue uptake and elimination. Our results suggest that SIM01 delivered in a liposomal dispersion is as effective as the raw formulation, something that would open new delivery routes and PDT applications.