We utilized the sol-gel reaction to prepare an immunoisolatable membrane for a microcapsule-shaped bioartificial pancreas. The membrane, derived from two precursors, 3-(aminopropyl)trimethoxysilane (APTrMOS) and tetramethoxysilane (TMOS), was formed onto calcium-alginate gel beads via electrostatic interaction. The molecular weight cutoff point of less than 150 000 required for immunoisolation was achieved at molar ratios ([APTrMOS]/[TMOS]) ranging from 0.60 to 2.40 with the amount of APTrMOS fixed at 3.40 mmol/(10 mL of calcium-alginate). When encapsulated in a membrane prepared at the molar ratio of 0.60, the islets contracted in volume and showed no response to stimulation by a high glucose concentration. However, islets in a membrane prepared at the molar ratio of 2.40 showed no contraction and responded to the glucose stimulation at almost the same level as free islets. These results demonstrated that the molar ratio of the precursors was a dominant factor affecting membrane permeability and the insulin secretion activity of the encapsulated islets.