Purpose: The purpose of this study was to determine the roles of alginate composition and gelling cations on bead swelling, which affects its durability. Method: Using a 2-channel droplet generator, microspheres were generated with 1.5% solutions of low viscosity high-mannuronic acid (LVM), medium viscosity high-mannuronic acid (MVM), low viscosity high-guluronic acid (LVG) and medium viscosity high-guluronic acid (MVG) alginate. They were gelled by cross-linking with 1.1% solution of either BaCl2 or CaCl2. The diameters of the microbeads were measured and recorded on day 0. The microbeads were subsequently washed and incubated in saline at 37 degrees C for 2 weeks with size assessment every 2 days. The data were normalized by calculation of the percentage change from control ( day 0) for all groups of microbeads. Results: Diameters of all beads were between 550-700 microns on day 0. Viscosity had no effect on swelling of Ba++- and Ca++- alginate microbeads. Ca++- alginate microbeads were more prone to swelling than the corresponding Ba++- alginate beads. High G-Ba++ beads had only a modest increase in size over time, in contrast to the high M-Ba++. Conclusion: Alginate composition and the gelling cation have significant effects on bead swelling.