Steroid/TH receptors mediate transcriptional induction of promoters containing hormone response elements (HREs) through an unclear mechanism that involves receptor binding to both hormone and a HRE. Here we demonstrate that both HRE binding and the transcriptional inducing activities of one member of this family, TH receptor, were markedly enhanced by heterodimerization with H-2RIIBP, a non-TH-binding member of the steroid hormone receptor superfamily. H-2RIIBP, the mouse homologue of human retinoic acid-related receptor, was shown to form stable heterodimers with the TH receptor either in solution or when bound to a TH response element. The results presented indicate that it might be necessary for the TH receptor or other members of this superfamily to have specific partners for heterodimer formation to elicit maximal hormone-specific gene regulation from particular HREs.