We synthesized a temperature-responsive polymer, N-(isopropylacrylamide)-methacrylic acid copolymer, to which poly-L-lysine was introduced. The synthesized polymer as well as the parent polymer showed reversible soluble-insoluble changes in response to temperature changes across the lower critical solution temperature at 32 °C in an aqueous solution. We found that the polymer efficiently captured acidic bio-macromolecules such as RNA, glycosaminoglycans and mucin-type glycoproteins in biological samples, and the captured molecules were recovered using aqueous NaCl solutions at high concentration. The target acidic molecules thus obtained will be employed for further studies such as structural analysis after brief desalting procedure. The proposed method does not require any chromatographic separations, but only needs a small volume of an aqueous salt solution for releasing captured molecules. Overall procedures are quite easy and simple, and are completed at least within 1 h. We show a few examples for capturing RNA and glycosaminoglycans from cultured cells using the polymer.