L,L-Ethylene dicysteine (EC) is a ligand used in nuclear medicine for the preparation of 99mTc-L,L-EC, a tracer agent for renal function studies. A capillary zone electrophoretic method is described which allows us to monitor the purity of ethylene dicysteine, for which no other suitable method has been reported to date. A fused-silica capillary (L = 70 cm, l = 62 cm, 75 microns i.d.) was used with 10 mM sodium tetraborate, pH 10.0, as the background electrolyte. The voltage applied was 20 kV and the temperature was 25 degrees C. Benzyl alcohol served as the internal standard. The limit of detection was 0.07% and the limit of quantitation was 0.15%.