The transcriptional regulation displayed by all-trans-retinoic acid (RA) and 9-cis-RA is mediated by their nuclear receptors, RARs and RXRs, which selectively form heterodimers in presence of a naturally occurring RA response element, βRARE. By a standardized procedure involving Superose-12 size-exclusion column chromatography, we have resolved RAR α - RXR α complex in the presence of βRARE and evaluated the optimal conditions for the heterodimer formation. The monomers exhibited higher affinity to form heterodimers than homodimers. Presence of ligand was not mandatory for dimerization of the receptor monomers. Studies on the effects of salt concentration on the stability of complexes showed that the heterodimers were stable in a range of 70-100 mM sodium or potassium salts, the complex was unstable at higher concentrations than 150 mM. The dimers were most stable at a pH range of 7.0-8.5 in solutions containing 80 mM sodium or potassium salts. Heterodimerization among the different receptor subtypes expressed similar properties with regard to ionic concentration and pH requirement.