Previous studies suggest that the amino-terminal ubiquitin-like (ubl) domain of Rad23 protein can recruit the proteasome for a stimulatory role during nucleotide excision repair in the yeastSaccharomyces cerevisiae. In this report, we show that the 19S regulatory complex of the yeast proteasome can affect nucleotide excision repair independently of Rad23 protein. Strains with mutations in 19S regulatory subunits (but not 20S subunits) of the proteasome promote partial recovery of nucleotide excision repair in vivo inrad23 deletion mutants, but not in other nucleotide excision repair-defective strains tested. In addition, a strain that expresses a temperature-degradable ATPase subunit of the 19S regulatory complex manifests a dramatically increased rate of nucleotide excision repair in vivo. These data indicate that the 19S regulatory complex of the 26S proteasome can negatively regulate the rate of nucleotide excision repair in yeast and suggest that Rad23 protein not only recruits the 19S regulatory complex, but also can mediate functional interactions between the 19S regulatory complex and the nucleotide excision repair machinery. The 19S regulatory complex of the yeast proteasome functions in nucleotide excision repair independent of proteolysis.