A series of halodibenzo-p-dioxins bearing the arylazide photolabile functional group were synthesized and tested as photoaffinity labels for the Ah receptor. 2-Azido-3-iodo-7,8-dibromodibenzo-p-dioxin (KD = 0.76 X 10(-9) M) was selected for radiosynthesis. Analysis of the 125I-photoaffinity-labelled proteins in mouse-liver cytosol by denaturing gel electrophoresis revealed two peptides which had apparent molecular masses of 95,000 and 70,000 daltons respectively, were labelled in an approximately 1:1 ratio and were selectively labelled at low concentrations of the photoaffinity ligand (0.05 KD = 0.04 X 10(-9) M). In addition, their labelling was inhibited by co-incubation with an excess of unlabelled ligand. On chromatographic separation under non-denaturing conditions, these two peptides co-migrated. These studies suggest that the Ah receptor in mouse liver cytosol is a heterodimer composed of two non-covalently bound peptides (95 K and 70 K) which each have a ligand binding site.