Both the human retinoic acid receptor alpha (hRAR alpha) and a constitutively active mutant (F318A) of the mouse retinoid X receptor alpha (mRXR alpha F318A) ligand-binding domains were separately overexpressed in Escherichia coli, copurified as a heterodimer in a two-step procedure, and cocrystallized with an RAR alpha-specific antagonist by using polyethylene glycol 10,000 as precipitant. The crystals grew in the hexagonal space group P6(1)22 displaying the unit cell parameters a = b = 116.6 Angstrom and c = 207.8 Angstrom. They diffracted X-ray to a limit of 2.2-Angstrom resolution. The asymmetric unit comprises one heterodimer and the crystal contains 60% solvent. The structure was determined by molecular replacement and is currently being refined.