Thiomalic acid (monomercaptosuccinic acid) was labeled with 99mTc without the use of an intermediary reducing agent. Tissue distribution studies in rats following the injection of 99mTc-tagged thiomalic acid (99mTc-TMA) showed 40-48% of the injected dose in the kidneys. Renal incorporation of this compound was influenced by various parameters such as pH, quantity of thiomalic acid, heating time and the preparation-injection interval. Scintigrams of a midline kidney slice showed that the 99mTc activity concentrated mainly in the renal cortex. As a proposed renal-imaging agent, 99mTc-TMA compared favorably with 99mTc-Sn-dimercaptosuccinate and 99mTc-penicillamine regarding the percent incorporation into the kidney was superior in this respect to 99mTc-Sn-glucoheptonate and 99mTc-Sn-diethylenetriamine pentaacetic acid. The 99mTc-TMA was also highly stable through 24 h. The reagent can be made available in kit form and is easily combined with 99mTc in 2 steps. The absence of stannous ion in the 99mTc-TMA complex should avoid the problem of interference with other procedures involving pertechnetate 99mTcO4- as the imaging agent.