A son1 mutant was isolated as a mutant showing synthetic lethality with nin1-1 which is defective in the p31 component of the regulatory subunit of the yeast 26S proteasome. son1delta showed a synthetic effect with sen3delta and sun1delta, both components of the 26S proteasome, and with cdc28-1N. The 26S proteasome was partially purified from the wild type yeast. The FPLC fractions were analyzed by Western blotting using anti-Son1p antibody and antibodies against some authentic subunits of the 26S proteasome, and we found that Son1p co-migrated with components of the 26S proteasome. The 26S proteasome containing fraction was immunoprecipitated with anti-Son1p antibody. The resultant precipitate contained Nin1p, Sun1p, TBP1, and the 20S proteasome. Combining genetic and biochemical results together, we concluded that Son1p is a component of the yeast 26S proteasome.