The pore sizes of various temperature-sensitive hydrogel beads have been estimated at different temperatures by a gel filtration method using several probe molecules of known molecular weight. The hydrogel beads were prepared by inverse suspension copolymerizations of a series of N-isopropylacrylamide (NIPAAm) and acrylamide (AAm) monomers with a small amount of cross-linker. These hydrogel beads exhibit LCST (lower critical solution temperature) behavior in aqueous solution; that is, expanding and swelling,when cooled below the LCST and shrinking and collapsing when heated above the LCST. The pore sizes of these temperature-sensitive hydrogel beads are affected significantly by both the temperature and the gel composition, but not much by cross-linker concentration.