Human urinary beta 2-microglobulin (beta 2m) and pa-pain-solubilized human histocompatibility antigen HLA-B7 were iodinated with iodogen and the sites of iodination determined. In the case of free urinary beta 2m, four of the six tyrosines were modified to some degree. Two of these were heavily iodinated (tyrosine-63 and -67) while two were lightly iodinated (tyrosine-10 and -26). In the case of beta 2m iodinated in the intact HLA-B7 complex, only one of these tyrosines was modified substantially (tyrosine-67). beta 2m iodinated at either of the two major sites exchanged into the HLA-B7 complex, whereas beta 2m iodinated at either of the two minor sites did not exchange at all. The relationship of these findings to the quaternary structure of HLA is discussed.