Despite active investigations, the structure of the thyrotropin (TSH) receptor, its organization and its interaction with putative anti-receptor antibodies remains unknown. We have undertaken photoaffinity cross-linking of TSH-binding sites on porcine thyroid plasma membranes with 125I-bTSH and the photoactive heterobifunctional cross-linker, N-hydroxysuccinimidyl-4-azidobenzoate. In the presence of reductant a single Mr 58,600 band was labelled, whereas 3 distinct bands (Mr 192,400, 86,000 and 59,600) were visualized when photolysed samples were subjected to sodium dodecyl sulphate polyacrylamide gel electrophoresis under non-reducing conditions. The intensities of these labelled bands were quantitatively decreased with increasing concentrations of native bTSH, but not by large amounts of insulin or human chorionic gonadotropin. Graves' disease IgG and an anti-TSH anti-idiotype IgG, both of which are putative anti-receptor antibodies, caused different inhibition patterns on non-reduced gels: anti-TSH anti-idiotype predominantly inhibited the labelling of the Mr 192,400 band (probably the holoreceptor), whereas Graves' IgG also inhibited labelling of lower M.W. bands. Both antibodies had much less influence, of the Mr = 58,600 band resolved in the presence of reductant.