This study investigated the new radiopharmaceutical, Tc-99m mercaptoacetyltriglycine (MAG3), as a possible alternative to Tc-99m DTPA and I-131 Hippuran for renal transplant evaluation. Tc-99m MAG3 and I-131 Hippuran were used sequentially in 19 consecutive patient studies and compared qualitatively and quantitatively. Serum creatinine values ranged from 1.6 mg/dL to 10.3 mg/dL (mean, 5.4 mg/dL). The Tc-99m MAG3 images and time-activity curves were of superior quality (3+) compared to I-131 Hippuran (1-2+). Quantitative analysis of the time to peak activity (Tmax) demonstrated a high correlation and no significant difference for Tc-99m MAG3 (mean, 11.4 min) compared to I-131 Hippuran (mean, 11.2 min). The percentage of peak activity retained at 20 minutes (T20) also showed a high correlation between the two agents; however, Tc-MAG3 showed a significantly slower clearance (T20 mean, 77%) than I-131 Hippuran (mean, 71%). Flow images were judged to be of good quality; however, the first-pass time-activity curves were often different from that seen with Tc-99m DTPA. Interpretation of delayed Tc-99m MAG3 images, for example to diagnose a slow urinary leak, were sometimes complicated by bowel clearance. In conclusion, Tc-99m MAG3 is superior to I-131 Hippuran for evaluation of renal transplant function. Tc-99m is the agent of choice for evaluation of renal transplant blood flow and function.