Abstract: Biomaterial such as an enzyme, cell, antigen or antibody is immobilized by covalent bonding to a substrate having an Si.sub.3 N.sub.4 surface containing Si--NH.sub.2 groups that provide reactive NH.sub.2 groups. A heterobifunctional cross-linking agent having an NH.sub.2 -reactive group and a biomaterial-reactive group is reacted with the NH.sub.2 groups of the surface and then with the biomaterial or the heterobifunctional cross-linking agent is reacted with the biomaterial and then with the NH.sub.2 groups of the surface. The Si.sub.3 N.sub.4 surface containing Si--NH.sub.2 groups can be formed by precipitating on a substrate a 10-1000 nm thick Si.sub.3 N.sub.4 layer from a SiH.sub.4 /NH.sub.3 mixture, and providing reactive NH.sub.2 groups on the surface of the layer by subjecting the surface to hydrolyzing cleaning.