Abstract: An implantable lead for use with an implantable pacemaker/cardioverter/defibrillator. Since irradiation from a radioisotope source is capable of inhibiting the growth of hyperproliferating cells as compared with normal cells, a radioisotope material which is incorporated into the lead can be used to decrease the rate of fibrotic growth. The radioisotope may be located inside the lead, alloyed into the metal from which the lead electrode or conductor is made, molded into rubber portions of the lead, or coated onto the conductor or electrode's surface. Beta emitting radioisotopes would be best suited as a radioactive material because of their comparatively short range of action within human tissue.