Several lines of evidence have indicated that the estrogen receptor (ER) can recruit the corepressors, nuclear receptor corepressor (NCoR) and silencing mediator of retinoid and thyroid receptors (SMRT), to target genes in the presence of tamoxifen, suggesting a possible role for NCoR/SMRT in regulating ER pharmacology. However, a tamoxifen-dependent, direct interaction between NCoR/SMRT and ER in vitro has not been demonstrated. To investigate the possible involvement of different corepressors in the actions of antiestrogen-bound ER, we have constructed a phage display library that expresses 23-amino acid peptides containing the canonical CoRNR box motif in an otherwise random background. Screening of the CoRNR box library with apo-ER or ER treated with tamoxifen or ICI 182,780 led to the isolation of peptides whose ability to interact with ER was influenced by the nature of the bound ligand. Using a series of ERalpha mutants, we found that helix 12 was not required for the binding of CoRNR box peptides, whereas disruption of helixes 3 and 5 had a marked effect on peptide binding. One mutant, ER-L372R, lost the ability to interact with CoRNR box-containing peptides without affecting its binding to LXXLL motif-containing peptides. The estradiol- and tamoxifen-mediated transcriptional activity of ER-L372R was dramatically increased by 11- and 3-fold, respectively, compared with that of wild-type ERalpha. The ICI 182,780-mediated repressional activity of this mutant was also reduced by 4-fold compared with that of wild-type ERalpha. These results suggest that leucine 372 may be an important part of the interaction surface on ER that is responsible for corepressor binding. In addition, our data suggest that corepressors, other than NCoR/SMRT, may be involved in ER signaling.