Blood samples were obtained periodically before and for about 3 mo after renal transplantation from a donor (mother) and a recipient (9-yr-old son with bilateral nephrectomy) for determination of serum protein binding of salicylate and sulfisoxazole. Ten days after the transplant, the recipient had an acute rejection episode which was treated successfully. The free fraction values of the drugs in serum of the recipient was considerably above normal before transplantation, decreased to normal within 2 days after transplantation, increased again several days before the rejection episode and remained elevated for about 50 days, and then returned to normal. Serum protein binding of the drugs in the serum of the donor was decreased for only a few days after transplantation and then returned to normal.