We attempted to purify ethyl docosahexaenoate (E-DHA) by the alcoholysis of fatty acid ethyl esters with lipase. Fatty acid ethyl esters originating from tuna oil (E-DHA content, 23 mol%; E-tuna-23) were used as starting materials, and Rhizopus delemar lipase immobilized on a ceramic carrier was used as a catalyst which acted only very weakly on E-DHA. Because the immobilized lipase did not exhibit the alcoholysis activity, it was activated by shaking at 30 degrees C for 24 h in the E-tuna-23/lauryl alcohol mixture to which water (2%) was added. The alcoholysis activity of the lipase increased markedly as a result of this pretreatment, but the hydrolysis activity also increased. The hydrolysis activity was completely repressed by repeating the reaction after transferring the immobilized enzyme into a fresh E-tuna-23/lauryl alcohol mixture without adding additional water. Several factors affecting the alcoholysis of E-tuna-23 were investigated to determine the optimum reaction conditions. When alcoholysis was conducted at 30 degrees C with shaking in a reaction mixture containing E-tuna-23/lauryl alcohol (1:3, mol/mol) and the activated lipase (4% of the mixture volume), E-DHA was efficiently enriched in the ethyl ester fraction. By alcoholyzing E-tuna-23 with lauryl alcohol for 50 h under these reaction conditions, the E-DHA content was increased from 23 mol% to 52 mol% in a 90% yield. In addition, when the fatty acid ethyl esters, of which the E-DHA contents were 45 mol% and 60 mol%, were alcoholyzed for 50 h, the contents of E-DHA were increased to 72 mol% and 83 mol%, respectively. In these reactions, the recovery of E-DHA in the ethyl ester fraction was greater than 90%. We termed this new reaction system selective alcoholysis because advantage of the fatty acid specificity of the lipase was taken in this reaction system. To investigate the stability of the immobilized lipase, continual batch reactions were carried out by replacing the reaction mixture with a fresh E-tuna-23/lauryl alcohol mixture every 24 h. The decrease in the extent of alcoholysis was only 15% even after the 47th batch reaction.