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 alpha- and beta-subunits. Further, assessment of the level of microheterogeneity of radiolabelled bTSH preparations can be achieved with these reversed-phase techniques.