A method for the partial purification of rat liver paraoxonase is presented. The method consists of the following steps: preparation of microsomes, solubilization with Triton X-100, adsorption on hydroxylapatite and chromatography on DEAE-52 cellulose. A partially purified preparation of rat liver paraoxonase has been obtained, showing a specific activity of 422 mU/mg with a yield of about 22% and a purification factor of 77-fold.