Proteasome plays an important role in the processing of antigenenic proteins for presentation by the MHC class I pathway. PA28 is an activator of the 20S proteasome, a large multisubunit complex involved in intracellular proteolysis. Although PA28 has been reported to be composed of two homologous subunits, alpha and beta;, arranged in alternating positions in a ring-shaped oligomer with a preferred stoichiometry of (alpha)3(beta)4, the packing structure of these subunit has not been clarified. It has been reported that a homo-oligomer of alpha or beta subunits of PA28 is incapable of activating 20S proteasome and alpha subunits are responsible for the substrate recognition. In view of the situation, we have made an attempt to reveal the packing structure of the subunits in hetero-oligomer of PA28 by utilizing a contrast variation method of SANS.