Abstract: A method and system are disclosed for externally treating human blood, with the objective of reducing the functioning lymphocyte population in the blood system of a human subject. According to the method, blood is withdrawn from the subject and passed through an ultraviolet radiation field in the presence of from about 1 nanogram to 100 micrograms per ml of blood, of a dissolved psoralen capable of forming photoadducts with DNA, to thereby effect covalent bonding between the psoralen and the nucleic acid of the lymphocytes, thereby altering the said nucleic acid and inhibiting the metabolic processes of the lymphocytes; and thereupon returning the irradiated blood to the subject. The withdrawn blood may be formed into an extracorporeal stream and flowed through a treatment station whereat the irradiation is effected, as for example by exposure to UV energy in the wave length range of from about 3200 to 4000 Angstroms; and such flow process may be conducted on a continuous basis.