A membrane suitable for conducting macromolecular fluid separations (e.g., protein filtration) is described. The membrane comprises crosslinked polymer formed by an acid catalyzed crosslinking reaction from a cellulosic polymer and a crosslinking agent. In a particular embodiment, the cellulosic polymer is one having substantial crosslinkable hydroxyl moiety content; and the crosslinking agent is one capable of releasing an electrophilic ion in an acidic solution, the electrophile capable of reacting with the hydroxyl moiety of the cellulosic polymer to effect the crosslinking thereof. Good results are obtained by using a multifunctional N-alkyloxy compound as the crosslinking agent.