Two-dimensional chymotryptic peptide maps of Lyt-1.1 and Lyt-1.2 from Lyt-1 congenic thymocytes labeled with 125I in the usual way before lysis did not significantly differ, but there was a characteristic difference between maps of Lyt-1.1 and Lyt-1.2 obtained from 125I-labeled solubilized membrane fragments. We conclude that the Lyt-1 locus of chromosome 19 includes the protein-structural gene for Lyt-1. This conclusion is further supported by evidence with 35S-cysteine-labeled thymocytes: Thus an early 62K intermediate form, and a 60K form from tunicamycin-treated cells devoid of N-linked and O-linked carbohydrates, were precipitated by Lyt-1 alloantibody, which implies that the allo-specificity of Lyt-1 glycoprotein (67K) resides partly or wholly in protein.