Transesterification of synthetic dimyristoyl phosphatidylcholine with oleic acid by commercial lipase preparations from Aspergillus niger and Rhizomucor miehei was studied in the presence and absence of solvent. A high-performance liquid chromatography method for determination of the modified phosphatidylcholine was developed. Under solvent-free conditions, transesterification could be carried out as efficiently as in toluene, and the degree of hydrolysis was lower than in toluene. Transesterification was influenced by the water content as well as by the fatty acid concentration in the reaction mixture. The optimum water content for transesterification in solvent-free reaction medium was higher than in toluene with both lipases. The yield of modified phosphatidylcholine increased, and the degree of hydrolysis decreased with increasing fatty acid concentration. The maximum yield of modified phospha-tidylcholine, 35% of the original phospholipid, was obtained with R. miehei lipase.