Iodination of ovine luteinizing hormone (LH) by the procedure of Sonoda and Schlamowitz (6) resulted in a preparation labeled almost exclusively on the LH–α subunit. When the iodination was carried out on the isolated subunits iodination of either subunit proceeded readily. The isolated LH–α subunit was iodinated approximately twice the quantity obtained with LH–β under identical conditions, which correlates with the relative amount of tyrosine in each subunit. These data are interpreted as evidence for “masking” of the two tyrosines in LH–β when bound to the –α subunit. (Endocrinology91: 317, 1972