Human serum arylesterase (EC 3.1.1.2) was purified over 400-fold with a recovery of 61-78% by single-step immuno-affinity chromatography using a monoclonal antibody, 12-1H8, as the adsorbent. The resultant preparation behaved as one component on SDS-PAGE with an apparent M(r) of 46,000, indicating its high homogeneity. Molecular weight was determined as 380,000 Da by HPLC on TSK-gel G-3000SW. The arylesterase molecule would thus appear to be comprised of octameric subunits. The purified protein hydrolyzed paraoxon. The present study suggests that a separate classification of EC 3.1.8.1 for its paraoxon hydrolyzing activity may not be appropriate to differentiate from arylesterase activity. In addition, the substrate for arylesterase is discussed in terms of its chemical structure.