Abstract: A method of producing a therapeutic effect upon a living organism endowed with a lymphatic system, comprising the steps of with drawing from said organism a biological fluid, wherein lymphocytes prevail as the cellular constituents, and introducing into said biological fluid a photo-active chemical agent soluble therein. The chemical agent has a specific affinity for the receptor sites in or on the fluid constituents, and is capable of being activated by ultraviolet radiation, and, when activated by said ultraviolet radiation, of forming photo-adducts with receptor sites of the fluid constituents, thereby ensuring chemical linkage between said photo-active chemical agent and said receptor sites. The method further includes irradiating the biological fluid, with the photo-active chemical agent introduced thereinto, by ultraviolet light, and introducing said biological fluid after said irradiation back into said lymphatic system. Irradiation may be effected endolymphatically.