<p>An adhesion preventive material comprising a crosslinkable polysaccharide derivative which has, incorporated in a polysaccharide side chain, at least one active ester group reactive with an active-hydrogen-containing group and which, upon contact with water under alkaline conditions, can form a crosslinked object through covalent bonding between the active ester group and an active-hydrogen-containing group. The treatment of the material to be conducted in preparation for the use predicted beforehand is easy and needs no special apparatus.</p>
