Humans are exposed to a range of xenobiotic esters used as pesticides. Hydrolysis by esterases present in liver microsomes, cytosol and blood is a major route for the detoxification of xenobiotic esters in humans. We have studied the subcellular localization and some biochemical properties of human liver paraoxonase in order to establish a correlation with serum enzyme and the relative importance of both activities in the toxicity of OP compounds.The subcellular localization of paraoxonase (POX) was determined by differential centrifugation. POX activity was determined spectrophotometrically using methods described elsewhere. Kinetic parameters (Km, Vmax), optimum pH and temperature and the effect of EDTA, calcium and some inhibitors were determined.POX activity was found mainly in the microsomal fraction of human liver, that was further used for the biochemical characterization. Human liver POX showed maximum activity at pH 10-10.5 and 37-40°C and was estimulated by NaCl 0.3 M. Km was 0.29 mM and Vmax 99.64 mU/ml. Activity was calcium dependent (optimum [Ca + + ] = 1 mM) and was abolished by EDTA and restored by adding calcium. POX activity was inhibited by mercurial compounds, barium, cobalt, lanthanum and zinc.