We studied the renal transplant blood flow using an ultrasonic contrast agent. The contrast medium was injected intravenously and produced blood flow enhancement for at least 60 seconds. Measurements were made from the recorded Doppler spectra. Fourteen renal transplant recipients were investigated by color Doppler echography. Blood flow in the interlobular artery could be analyzed in all patients using a contrast, agent, as compared with 2 patients (14%) by a conventional method. The average maximum and minimum flow velocity of the interlobular arteries were 0.15 m/sec and 0.04 m/sec. In conclusion, an ultrasonic contrast agent is useful to accurately evaluate peripheral blood supply in renal transplants.