Technetium-99m mercaptoacetyltriglycine (MAG3) has been used extensively as a renal function agent for several years. Radioimpurities in the MAG3 kit preparation are not only concentrated and excreted in the kidneys. Therefore proper quality control for renal studies is even more important to make possible a rapid and adequate interpretation of diagnostic studies. The standard method to determine the radiochemical purity is high-performance liquid chromatography (HPLC), which is time consuming and expensive. We set up a new simple thin-layer chromatography (TLC) method based on instant thin-layer silica gel strips as stationary phase for checking the main impurities, free pertechnetate and reduced hydrolysed 99mTc-colloid. Comparison of TLC and HPLC results showed no significant differences (t-test, P < 0.05); the correlation between the results obtained with the two methods in respect of the free pertechnetate content was excellent (r = 0.99913). The TLC method was also efficient in determining the percentage of 99mTc-colloid. The main impurity found in 37 routine preparations was free pertechnetate; the mean radiochemical purity was 97.95%. The time required to perform the analysis was less than 20 min. The new TLC system is a cheap, simple, fast, reliable and accurate method for the quality control of the MAG3 kit preparation, and is especially suitable for routine use.