The 26S proteasome, a protein complex consisting of a 20S proteasome and a pair of 19S regulatory particles (RP), is involved in ATP-dependent proteolysis in eukaryotes. In yeast, the RP contains six different ATPase subunits and, at least, 11 non-ATPase subunits. In this study, we identified the rice homologs of yeast RP subunit genes from the rice expressed sequence tag (EST) library. The complete nucleotide sequences of the homologs for five ATPase subunits, OsRpt1, OsRpt2, OsRpt4, OsRpt5 and OsRpt6, and five non-ATPase subunits, OsRpn7, OsRpn8, OsRpn10, OsRpn11 and OsRpn12, and the partial sequences of one ATPase subunit, OsRpt3, and six non-ATPase subunits, OsRpn1, OsRpn2, OsRpn3, OsRpn5, OsRpn6 and OsRpn9, were determined. Gene homologs of four ATPase subunits, OsRpt1, OsRpt2, OsRpt4 and OsRpt5, and three non- ATPase subunits, OsRpn1, OsRpn2 and OsRpn9, were found to be encoded by duplicated genes. The rice RP was purified by immunoaffinity chromatography with a Protein A column immobilized antibody against rice 20S proteasome, and the subunit composition was determined. The homologs obtained from the rice EST library were identified as genes encoding subunits of RP purified from rice, including the both products of duplicated genes by using electrospray ionization quadrupole time-of-flight mass spectrometry. Post-translational modifications and processing in rice RP subunits were also identified. Various types of RP complex with different subunit compositions are present in rice cells, suggesting the multiple functions of rice proteasome.