Cysteine was chelated with 99mTc and/or 99Tc in a freeze-dried kit containing Tin(II) ions, and the yellow 99m/99Tc-cysteine complex (complex I) was separated to study the biodistribution. Comparison of the distribution of 99m/99Tc-cysteine and 99mTc-cysteine complexes was made in rats and mice. The renal excretion patterns were studied in rats in the presence and absence of the renal tubular transport inhibitor 2,4-dinitrophenol. The carrier of Tc-cysteine complex in the blood and also the radioactive compounds in the urine were studied by HPGFC and SDS-electrophoresis. The kidney was confirmed as the target organ; serum albumin serves as a carrier for transport of Tc-cysteine complex to the kidney. The Tc-cysteine complex was the primary form in excreta, and glomerular filtration was the dominant excretory pathway.