Protein crystals generally are stable solid protein assemblies. Certain protein crystals are suitable for use as nanovessels for immobilizing metal complexes. Here we report the preparation of ruthenium carbonyl-incorporated cross-linked hen egg white lysozyme crystals (Ru center dot CL-HEWL). Ru center dot CL-HEWL retains a Ru carbonyl moiety that can release CO, although a composite of Ru carbonyl-HEWL dissolved in buffer solution (Ru center dot HEWL) does not release CO. We found that treatment of cells with Ru center dot CL-HEWL significantly increased nuclear factor kappa B (NF-kappa B) activity as a cellular response to CO. These results demonstrate that Ru center dot CL-HEWL has potential for use as an artificial extracellular scaffold suitable for transport and release of a gas molecule.