We have begun to examine methods for biochemically modifying orthopedic biomaterials by covalently immobilizing peptides, proteins, and enzymes. The surfaces of Co-Cr-Mo samples were first silanized using γ-aminopropyltriethoxysilane (APS), which interacts with surface hydroxyl moieties. Derivatization of biomaterial samples with solutions of APS in acetone produced a concentration-dependent number of reactive NH2 groups. The silane layers became unstable and were easily disrupted at concentrations above 2% APS. The enzyme, trypsin, was then coupled to the alkylamine-derivatized samples by formation of Schiff's base linkages via glutaraldehyde. Enzymatic activity of trypsin-conjugated Co-Cr-Mo samples was quantified by determining cleavage of the substrate BAEE. Both alkoxysilane-treated and untreated samples bound trypsin in an active state. When treated with 5 M guanidine hydrochloride to elute noncovalently bound protein, however, nearly all of the trypsin was removed from or rendered inactive on samples either not treated with APS or derivatized with low (<0.5%) concentrations. On the contrary, 1 and 2% APS-treated samples retained appreciable enzyme activity. Approximately 27 and 35% residual trypsin activity was measured on the 1 and 2% APSderivatized Co-Cr-Mo samples, respectively. Active trypsin remained immobilized on the APS-derivatized substrates when at least 35 free amino groups per nm2 of nominal surface area were available for protein coupling.