The in vivo antitumour activity of the natural photosensitizer hypericin was evaluated. Athymic nude mice xenografted with A431 cells were intraperitoneally administered with different hypericin doses and the tumours were locally irradiated 2 h later with white light (180 J/cm2) using a cold light source. When treatment was started one day after tumour inoculation, a dose-dependent antitumour effect was observed in light-treated animals. Complete inhibition of the tumour growth was achieved with 2.5 mg/kg hypericin. When the efficacy of a single hypericin dose (5 mg/kg) followed by a single light treatment on established tumours (60 mm3) was investigated, an 80% reduction in tumour mass was seen. Furthermore, an accumulation of the photosensitizer in A431 xenografts was observed after local light irradiation. Our results strongly suggest that hypericin holds promise as a new, safe, efficient and selective PDT photosensitizer.