Animal tissues used in the manufacture of medical devices require chemical treatments to improve durability. A number of techniques have been proposed to preserve such collagenous biological tissues. This study reports a new strategy for the crosslinking of bovine pericardium with a blocked polyurethane prepolymer (PUP) in aqueous medium. The decrease in amino group content (70 %) and corresponding increase in shrinkage temperature (13 °C) when native tissue was treated with PUP, indicate the participation of tissue-amines in the stabilization process. The resistance toward degradation by collagenase in PUP-treated tissue indicate a in vitro stability similar to the glutaraldehyde-crosslinked tissue. It can conclude that the isocyanate-blocked PUP is effective agent for the aqueous fixation of biological tissues of biomedical interest.