The chromatographic behaviour of bovine and human thyrotropin, radiolabelled stoichiometrically with lactoperoxidase, on octadecylsilicas and other reversed-phase n-alkylsilicas has been investigated. As part of this investigation the effects of a variety of elution systems on resolution and recovery have been examined. Analysis of the tryptic peptides of radioiodinated bovine thyrotropin (bTSH) preparations by reversed-phase high-performance liquid chromatographic mapping methods resulted in the separation of more than fourteen major radioactive peptide components. The data indicate that bTSH is radioiodinated with unequal incorporation into both the α- and the β-subunits. Further, assessment of the level of microheterogeneity of radiolabelled bTSH preparations can be achieved with these reversed-phase techniques.