A novel microcapsule membrane composed of aminopropyl-silicate was synthesized on Ca-alginate gel beads from 3-aminopropyltrimethoxysilane (APTrMOS) and tetramethoxysilane (TMOS) by the sol-gel method. Preparation conditions such as the molar ratio of APTrMOS to TMOS, the sequence of addition and the contact time of each precursor were examined to give the membrane immunoisolatable permeability for microcapsule-shaped bioartificial organs. The permeability of the membrane decreased with an increase of up to 2.4 in the molar ratio, with gamma-globulin being completely rejected at this ratio. Addition of APTrMOS prior to TMOS and allowing 1 min of contact with the Ca-alginate gel beads suspended in n-hexane was effective for rejection of gamma-globulin. The optimized aminopropyl-silicate membrane was estimated to give a molecular weight cut-off point of less than 150 kDa, as required for immunoisolation, and an excellent permeability to low molecular weight substances such as glucose and oxygen. Our data show that the permeability of the aminopropyl-silicate membrane, that is the microscopic structure of the membrane, is strongly controllable by altering preparation conditions.