An alginate/aminopropyl-silicate/alginate membrane was prepared on calcium-alginate gel beads via electrostatic interaction and sol-gel processes. The effects of the chemical composition and the molecular weight (MW) of the core alginate on the permeability of the alginate/aminopropyl-silicate/alginate membrane were investigated for five different types of alginates. Our data shows that an immunoisolatable MW cut-off point was achieved for the membranes formed on the alginates, MW = 7.0 x 10(4) with molar ratio of M/G (mannuronic acid to guluronic acid), M/G = 1.30, MW = 1.1 X 10(5) with M/G = 1.30, and MW = 7.0 x 10(4) with M/G = 0.65. For these three membranes, the membrane templated by the alginate of MW = 1.1 x 10(5) had a higher MW cut-off point than that templated by the alginate of MW = 7.0 x 10(4). In addition, the MW cut-off point increased with an increase in the content of guluronic acid. The membranes formed on the alginates of MW = 2.0 x 10(4) with M/G = 1.30, and MW = 7.0 x 10(4) with M/G = 2.25 did not have an immumoisolatable MW cut-off point.