Using paraoxon and pirimiphos-methyl as substrates, much of the "A'-esterase activity of sheep serum was found to be in the high-density lipoprotein (HDL) fraction. A method was developed for the partial purification of "A'-esterases by the preparation of a lipoprotein fraction, followed by preparative polyacrylamide gel electrophoresis. The properties of the partially purified preparations of "A'-esterase were studied. Although four different preparations all contained a major protein unit, which resembled the core protein of HDL, there was evidence of differences between preparations with regard to substrate specificity, suggesting the existence of multiple enzyme forms. Gel filtration of serum samples indicated that paraoxonase activity is expressed by proteins with mol. wts greater than 200,000, strongly suggesting that the "A'-esterase activity of the lipoprotein fraction is present in one or more forms of HDL2. The dependence of "A'-esterase activity upon Ca2+ and the problem of esterase classification are discussed.