A thin film of white blood cells is irradiated with ultraviolet radiation predominately of a wavelength of 280 to 320 nanometers, and preferably at an intensity of 4 to 15 milliwatts per square cm. At such a radiation intensity it is not necessary to agitate the thin film of white blood cells to accomplish the purposes of this invention. The white blood cells so processed substantially lose their capability to set off an immune reaction in an alloimmunized patient. The white blood cells may preferably be placed into such film in a flat, flexible bag made of poly(ethylene-vinyl acetate) plastic, with the flat, flexible bag being stretched in a direction normal to the path of ultraviolet radiation.