In a recent study it is suggested that a short peptide fragment within the estrogen receptor ligand-binding and dimerization domain might act as a constitutively active dimerization motif [Lees, J. A., Fawell, S. E., White, R. & Parker, M. G. (1990) Mol. Cell. Biol. 10, 5529-5531]. We used NMR and CD spectroscopies to characterize the structure and biophysical properties of a synthetic peptide comprising residues Thr500-His528 of the mouse estrogen receptor, including the putative dimerization motif. We found that residues Leu501-Asn523 form a nascent helix in water solution, whereas the C-terminal (Lys524-His528) has no propensity for alpha-helical conformation. We found no evidence for a strong homodimerization activity of the peptide. However, we observed concentration-dependent NMR chemical shifts of several residues that would be located on the same face of an alpha-helix. This observation suggests a weak, but specific dimerization/oligomerization of the peptide at millimolar concentrations.