The modification of anhydrous butterfat via interesterification reactions with oleic acid catalyzed by a lipase from Mucor circinelloides immobilized by adsorption onto hydrophobic hollow fibers is described A reasonable degree of incorporation of free (externally added) oleic acid into the triacylglycerols of butterfat has been ;achieved while short-chain fatty acid residues remained virtually unaffected. Total saturated triacylglycerols decreased by 27%, and triacylglycerols with 32-44 acyl carbons (which contained two oi. three lauric, myristic, or palmitic acid residues) decreased by 33%. Total monoene and polyene triacylglycerols increased by 21% and 17%, respectively. The triacylglycerols (TAG) of interesterified butterfat had approximately 27% more oleic acid residues and approximately; 8% less lauric, 6% less myristic, and 6% less palmitic acid residues than those of the original butterfat; the fraction of low-melting TAG peak increased by 19% whereas that of high-melting TAG decreased by 83%. Although a certain degree of butterfat hydrolysis was observed, enzymatic acidolysis was technically feasible and able to produce a modified butterfat with a stronger nutraceutical character.