The immobilization of aminoacylase (N-acylamino acid amidohydrolase, EC 3.5.1.14) was investigated by using tannin immobilized on aminohexyl cellulose. The most active immobilized aminoacylase was obtained when aminoacylase was adsorbed to the immobilized tannin in a weak alkaline medium containing sodium chloride and n-butanol at 37 degrees C. The activity of the immobilized tannin-aminoacylase complex per unit volume was five times higher than that of the DEAE-Sephadex-aminoacylase complex used for industrial production of L-amino acids in our plants. The half-life of the immobilized tannin-aminoacylase complex was 20 days under continuous operation at a high concentration of substrate; on the contrary, that of the DEAE-Sephadex-aminoacylase complex was 0.5 days.