[en] Upon binding a ligand such as 2,3,7,8-tetrachorodibenzo-p-dioxin, the Ah receptor (AhR) apparently translocates into the nucleus and binds to specific DNA sequences. The addition of the photoaffinity ligand 2-azido-3[¹²⁵I]iodo-7,8-dibromodibenzo-p-dioxin to Hepa 1 cells in culture results in the specific labeling of the AhR. Both the cytosolic fraction and nuclear extract were isolated and subjected to sucrose density gradient analysis. The cytosolic fraction contained a single 9S peak of photoaffinity-labeled AhR. The nuclear extract contained both a 6S and a 9S peak. Both the nuclear and cytosolic forms of the photoaffinity-labeled AhR were subjected to chemical cross-linking with dimethyl pimelimidate and analyzed by SDS-PAGE. Cross-linking of the cytosolic form of the AhR revealed monomeric (97 kDa), dimeric (190 kDa), trimeric (280 kDa), and tetrameric (330 kDa) complexes. Cross-linking of the photoaffinity-labeled nuclear form of the AhR resulted in monomeric (97 kDa), and dimeric (184 kDa) complexes. In a time course of exposure to the cross-linking reagent, the largest forms given above become the predominant AhR forms in each extract. The 330 kDa cytosolic species apparently consists of a 97 kDa AhR, a dimer of 90 kDa heat shock protein and a 50 kDa protein. Previous work has shown that AhR is bound to HSP90 in cytosolic extracts. The 184 kDa nuclear AhR consists of a 97 kDa AhR, and an 87 kDa protein