Polymer-based monoliths with interconnected porous structure have attracted much attention as a high-performance stationary phase for online digestion liquid chromatography-mass spectrometry (LC-MS) system. In this study, a poly(glycidyl methacrylate-co-methyl methacrylate) (PGM) monolith prepared via thermally induced phase separation (TIPS) was used as a solid support to covalently immobilize pepsin. The PGM monolith was modified with aminoacetal to yield an aldehyde-bearing (PGM-CHO) monolith. Pepsin was immobilized onto the PGM-CHO monolith via reductive amination. The immobilized pepsin showed better pH and thermal stability compared with free pepsin. Furthermore, the PGM-CHO monolith modified with pepsin was applied for online protein digestion followed by LC-MS and LC-MS/MS analyses. As a result, a larger number of peptides are reproducibly identified compared to those by polystyrene/divinylbenzene particle (POROS)-based online pepsin column. (C) 2014, The Society for Biotechnology, Japan. All rights reserved.