Graft-versus-host disease (GVHD) is a lethal complication of allogeneic haematopoietic stem cell transplantation which limits its application. Psoralen was utilized in the treatment of GVHD as a second-line method, which is also known as extracorporeal photochemotherapy (ECP). In the process of ECP, mononuclear cells must be isolated from the body in advance. Then a photosensitizer, 8-methoxypsoralen (8-MOP, 200 μg/L for the final concentration), would be added to the cell suspension before its exposure to ultraviolet A [UVA; 365 nm, 2J/cm2]. The disposed lymphocytes re-infused into the body account for 5% to 15% of the lymphocytes in the body. The process of ECP is complicated, expensive and very labour intensive, which limits its popularity. We hypothesized that psoralen should be activated by ultraviolet A, and should be kept in activation for a relatively long time before it gets in contact with mononuclear cells. This kind of psoralen is called previously activated psoralen (PAP), which may have the same effects on GVHD as ECP, but would be much easier and economical to work with.