Foslip is a recently designed third generation photosensitiser based on unilamellar dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylglycerol (DPPC/DPPG) liposomal formulations of meta-tetra(hydroxyphenyl)chlorine (mTHPC). The present study investigates Foslip behaviour and its photodynamic efficiency in EMT6 xenografted nude mice at different times following i.v. administration of 0.3 mg kg(-1) mTHPC in a Foslip formulation. Plasma pharmacokinetics and biodistribution were studied by high performance liquid chromatography and were described by a three compartments analysis with half-lifes of 0.13, 4.31 and 35.7 h. The highest tumour to muscle ratios were observed at 6 and 15 h post-administration. Intratumoral distribution was carried out using two photon excitation confocal microscopy. Progressive efflux from the vascular compartment was noted in favour of tumour parenchyma, which was almost completed at 15 h. The best tumour response was obtained for a drug-light interval of 6 h, interval for which mTHPC was present in both endothelial and parenchyma cells. Tumour and plasma concentrations however were far below their maximal values. Based on these observations, we assume that the presence of mTHPC in both vasculature and tumour cells is required for optimal PDT efficacy.