(+/-)-trans-3-(4-Methoxyphenyl)glycidic acid methyl ester [(+/-)-MPGM], a key intermediate in the synthesis of diltiazem hydrochloride, was efficiently hydrolyzed by lipase from Serratia marcescens Sr41 8000 immobilized on a hollow-fiber ultrafiltration membrane. The lipase was immobilized on a spongy layer of the shell side of the membrane by means of physical adsorption. Asymmetric hydrolysis was carried out semi-continuously at 22-degrees-C in a membrane reactor circulating both toluene solution containing (+/-)-MPGM in the shell loop and aqueous solution (pH 8.5) in the lumen loop. (+)-MPGM was hydrolyzed to (+)-(2S, 3R)-3-(4-methoxyphenyl)glycidic acid and methanol. p-Methoxyphenylacetaldehyde derived from the glycidic acid was accumulated in the toluene phase and inhibited the enzyme reaction. The inhibition, however, was suppressed by the addition of sodium hydrogen sulfite to the aqueous phase, because the aldehyde and sodium hydrogen sulfite formed an adduct and the adduct was transferred to the aqueous phase with the progress of enzyme reaction. The lipase immobilized on the hydrophilic membrane exhibited a high stability : the half-life of enzyme activity at 22-degrees-C was 127 h, which was about 30-fold that in the conventional emulsion reactor. The maximum value of the velocity constant (k(+)) for the hydrolysis of (+)-MPGM was 0.25 h-1 under the condition of immobilization of 1.6 x 10(5) units of lipase per m2. Since reaction and product separation were achieved simultaneously, crystalline (-)-MPGM with a high yield of 40-43% and optical purity of 100% enantiomeric excess [e.e.] was obtained through six runs by concentration of the toluene phase after the reaction.