Fractionation of rat liver cytosol, using as an affinity reagent the DNA recognition sequence for the liganded aromatic hydrocarbon (Ah) receptor, enriches for proteins that are about 110, 106, 98/96, 57, and 54 kDa in size. The proteins display 2,3,7,8-tetrachlorodibenzo-p-dioxin-dependent, DNA sequence-specific binding that is characteristic of the liganded Ah receptor. Immunological studies imply: 1. That the 98/96-kDa protein is the Ah receptor nuclear translocator (Arnt); 2. That the 110- and 106-kDa proteins are not immunologically related either to each other or to Arnt; and 3. That the 110-, the 106-kDa, and the Arnt proteins are members of a multicomponent protein complex. cDNA cloning studies indicate that the 106-kDa protein is the rat Ah receptor. The N-terminal 384 amino acids of the rat receptor show substantial sequence homology to the mouse and human Ah receptor. The sequence conservation across species imputes functional importance to this region of the receptor. In contrast, the remainder of the protein is substantially less well conserved among rat, mouse, and human. The tissue distribution of Ah receptor mRNA is consistent with previous studies of the distribution of the receptor protein. Our findings demonstrate the use of DNA recognition site chromatography for purification of the Ah receptor and imply that the ligand receptor binds to DNA as part of a multiprotein complex.