We have expressed recombinant human vitamin D receptor and its ligand-binding domain in Spodoptera frugiperda (Sf9) insect cells with a 30-litre bioreactor. Both proteins were purified to apparent homogeneity with yields of 0.5-3.5 mg/l. Gel-filtration analyses indicated that the purified human vitamin D receptor and its ligand-binding domain were present as monomers in solution. The purified vitamin D receptor and its ligand-binding domain were demonstrated to bind 1alpha,25-dihydroxyvitamin D(3) with high affinity, the K(d) values ranging from 0.9 to 1.2 nM. Neutron scattering studies of the ligand-binding domain demonstrated that the samples are homogeneous and contain monomeric species of polypeptides. The purified vitamin D receptor binds to the vitamin D response elements of osteopontin and osteocalcin genes as a homodimer or as a heterodimer with the retinoid X receptor-alphaDeltaAB and we were able to purify these complexes in quantities sufficient for crystallization studies. The results indicate that we can produce biologically active human vitamin D receptor and its ligand-binding domain in insect cells and purify them for functional and structural studies.