Transamidinase (EC 2.1.4.1) was purified from the cytosol of Sprague-Dawley rat placenta by ammonium sulfate fractionation, and by DEAE-Sephacel, hydroxyapatite, arginine-Sepharose 4B, and LC Bio-Sil SEC chromatography. The criterion of enzyme purity was its migration as a single band in native and sodium dodecyl sulfate polyacrylamide gel electrophoresis. Transamidinase is a dimer with a molecular weight of 110,000. The enzyme migrates on isoelectric focusing with a pI of 5.6. Rat placenta transamidinase was specific for L-arginine and glycine, and the $K_m$ values for L-arginine and glycine were 1.3 mM and 0.19 mM, respectively. The optimum pH and temperature were respectively 8.3 and $50^{circ}C$. Transamidinase has no hydrolase activity and it produced appropriate amounts of ornithine for putrescine formation in the placenta, which was devoid of arginase activity.