Several chromatographic methods have been used for determining the radiochemical purity of 99m Tc-diethyl HIDA (Solco HIDA). Good separation of 99mTc-HIDA from 99mTc-hydrolyzate and nonreduced 99mTcO4- was obtained in a short time using Gelman ITLC (SA) and 2N NaC1 as the mobile phase. In vitro stability was followed by the same method. It was found that the presence of oxygen from air enhances the percentage of 99mTcO4- in the preparation. Formation of a chelate of 99mTc-HIDA with a high stability constant was established by using Sephadex G-25 filtration. The degree of binding of 99mTc-diethyl HIDA to blood plasma proteins, examined by the same method. In vivo stability was examined by analyzing bile and urine, which were found to contain 99mTc-HIDA chelate. Biodistribution of the preparation was also investigated in experimental animals.