Twenty-seven consecutive recipients of cadaveric kidneys, including five with pre-existing warm cytotoxic antibodies, were treated with thoracic duct drainage before and after transplantation. Fourteen patients who had lymph drainage for 26 to 58 days before transplantation had minor cytotoxic antibody responses after grafting, even if the antibodies had been present before therapy. Only one of the 14 recipients had any rejection during the follow-up periods of one to six months. There were two deaths. The 13 patients pretreated for 17 to 23 days exhibited stronger cytotoxic antibody responsiveness, and five of these recipients had significant rejections of which four were reversible. One of the latter 13 patients died. These clinical and immunologic studies have established the value and have defined the appropriate timing of preoperative thoracic duct drainage in kidney transplantation. They have also directed attention to the rationale andthe probable value of using other immunosuppressive methods for preparatory host conditioning instead of beginning such therapy at the time of transplantation.