Physico-chemical characterization of human renal dipeptidase was carried out. It was a glycoprotein with a subunit MW of approximately 47,700 dalton. The pH optimum was at 8 and its stable conformation was retained between pH 5 and 12. The kinetic parameters determined with imipenem, a novel β-lactam antibiotic, were Vmax, 5.21 μmol/min/mg; Km, 4.35 mM; and Ki with cilastatin, 0.25 μM. Cilastatin demonstrated reversible competitive inhibition.