Four patients with established life-threatening infections suffered rejection episodes on postrenal transplant days 17, 25, 228, and 351 associated with markedly reduced immunosuppressive therapy. In three patients, azathioprine had been discontinued and the fourth was receiving 25 mg daily. Prednisone was reduced to 10 to 15 mg/day in three patients and the fourth held constant at 30 mg/day. Antilymphocyte globulin was discontinued in all patients. Radiation was administered to prolong the interval before nephrectomy in two critically ill patients. However, in all patients, rejection was completely reversed with external cobalt radiation alone, 150 roentgens per treatment and a total of 300 to 900 R per rejection. Each patient recovered and has normal renal function. Our experience indicates that radiation alone may be used successfully to treat rejection episodes in patients with life-threatening infections when other immunosuppressive measures have been discontinued or markedly reduced.