The chemical construction of dipersed CdS nanoparticles within the polypeptide cage of apoferritin is demonstrated to be possible by a multistep synthetic route, as shown schematically in the Figure. Ferritin-induced nucleation of CdS clusters (squares) is followed by reaction with excess sulfide to give CdS-ferritin. Such biocompatible nanoparticles might find uses as tunable photochemical agents for diagnosis, sensing, and drug delivery.