Benzoporohyrin derivative monoacid ring A (BPD) is a lipophilic photosensitizer with a maximum absorption peak at 690 nm. When liposomally formulated, it distributes in human plasma almost exclusively to the lipoprotein fraction. In experimental animals and humans, it demonstrates good selectivity for tumors as well as other hyperproliferative tissues or cells. In experimental animal tumour models we have found maximum selectivity between tumour and normal surrounding tissue to occur within the first hour following intravenous administration, providing an opportunity for very early PDT following injection. Early clinical trial results from treatment of patients with cutaneous cancerous lesions have shown encouraging efficacy at levels of drug between 0.35 and 0.5 mg/kg. In addition, studies on patients with psoriasis indicate that BPD and light may be used to clear psoriatic plaques.