The 24 subunit heat-shock protein from Methanococcus jannaschii has been genetically altered to display a dodecapeptide on the interior surface that has a strong binding affinity to the L1(0) phase of CoPt (CP_Hsp). Reaction of Co-II and Pt-II salts at 65 degrees C under reducing conditions results in the formation of a stable CoPt mineral encapsulated within the protein cage. Metallic particles commensurate in size with the interior dimensions of the protein cage (6.5 +/- 1.3 nm) have been imaged by transmission electron microscopy and are shown to be surrounded by the intact protein cage. Magnetic measurements performed on the encapsulated nanoparticles exhibit room-temperature hysteresis on the order of 150 G (1 G = 10(-4) T) prior to annealing and 610 G after annealing at 650 degrees C.