Purified preparations of bovine plasma arylesterase were obtained by isoelectric focusing of enzyme prepared by (NH4)2SO4 fractionation of plasma and chromatography on DEAE-cellulose and Sephadex G-200. Although the high-density-lipoprotein fraction (HDL2) of serum provides an alternative source of enzyme, the enzymic activity of preparations made from it is much less stable. The purified arylesterase preparation has a molecular weight of 440000 and a partial specific volume of 0.91 ml/g, properties indistinguishable from those of the less highly purified enzyme. Extraction with acetone and ether removes neutral lipids from the enzyme, but the resulting lipid-depleted preparation retains most of the phospholipid present initially. A partial specific volume of 0.81 ml/g and a minimum molecular weight of approx. 100000 were determined for the lipid-depleted preparations of arylesterase. The present results support the concept of bovine plasma arylesterase as a lipoprotein in its own right, rather than as an enzymic polypeptide that is loosely associated with the HDL2 fraction of serum.