Renal transplant uptake of Tc-99m-tin colloid was evaluated in 26 patients. Seventy-seven examinations were performed comparing transplant with bone marrow activity, clinical and/or pathological diagnosis. There were 13 instances of acute rejection; 7 of these exhibited slight uptake of radiocolloid in the renal transplant, 1 had marked uptake, and 5 had no evidence of uptake. There were 7 instances of chronic rejection; 5 of which demonstrated marked transplant uptake of radiocolloid, 1 had slight uptake, and 1 had no evidence of uptake. There were 2 instances of acute tubular necrosis and 55 instances of normal transplant function, but none of these exhibited transplant uptake of radiocolloid. From the result, the uptake of Tc-99m-tin colloid by renal transplant appears to signal rejection as long as the vascular supply is not severely compromised. Acute rejection may be represented by slight radiocolloid uptake, and chronic rejection by marked uptake when compared to bone marrow activity.