The fluorescence kinetics of a new photosensitizer for photodynamic therapy, di-l-arginine protoporphyrinate (PP(Arg)2), was studied in the skin of healthy mice. Furthermore, induction of necrosis in WiDr human colon adenocarcinoma xenografts in athymic nude mice was studied after photodynamic therapy (PDT) with PP(Arg)2. After intravenous administration of PP(Arg)2 maximal fluorescence was reached after 72h in normal mouse skin. Complete elimination of the drug from the mouse skin was not found even after 32 days. Exposure of WiDr tumours in mice to red light (λ=632nm, fluence 150J/cm(2), fluence rate 250mW/cm(2)) 24 and 72h after intravenous administration of 10mg/kg of PP(Arg)2 caused extensive tumour necrosis. Epidermal damage and infiltration of inflammatory cells was seen 24h after light exposure but not after 72h.