Cellular uptake of rigid and flexible supramolecular hemoprotein assemblies formed by cytochrome b(562) was investigated. Multivalent cell-penetrating tags on the protein surfaces of the assemblies improve the efficiency of cellular uptake into HeLa cells in a manner which depends on the number of arginine residues in the tags and the main-chain rigidity of the assemblies. A rigid protein assembly with an arginine tetramer tag shows 10-fold higher cellular uptake efficiency than a corresponding assembly without an arginine tag.