Oxidative-acid-treated nanodiamonds exhibit high affinity for proteins, a property well suited for immobilization of enzymes for biotechnological application. Using lysozyme as an example, this work demonstrates that the enzyme can retain much of its activity after physical adsorption to the surfaces of 100-nm diamond crystallites. The activity relative to that of free lysozyme in solution is ∼ 60% at the maximum surface coverage of 50% and pH 5. While the enzymatic activity decreases as the surface coverage is lowered, it can be recovered by blocking the empty sites on the surface with supplementary proteins such as cytochrome c to create a more “crowded” environment. A relative activity up to 70% can be attained at a partial coverage of 20%.