Extracorporeal PHOTOPHERESIS is a form of apheresis therapy in which the collected mononuclear cells are treated with a photosensitizing agent (8-methoxypsoralen). The formulation is then exposed to UVA rays and the cells processed in this manner are directly reinfused into the patient's blood circulation. The procedure takes 3 hours and is conducted on two consecutive days, 14 days apart, hence, twice a month. It is used for treating patients with rheumatoid arthritis, systemic sclerosis, systemic lupus and so on. The most notable indication, however, is the treatment and prevention of an acute and chronic form of graft-versus-host reaction due to the transplantation of stem cells or organs.