Guinea pig Ia molecules bearing the markers 3, 5 and 7 (strain 13) and 4, 5 (strain 2) were isolated from spleen and lymph node cells. In the molecules bearing the 7 and the 4, 5 markers, the 25,000 and 33,000 MW polypeptide chains are linked by disulfide bonds. The molecules bearing the 3, 5 markers do not have polypeptide chains linked by disulfide bonds. These glycoproteins were metabolically labeled with tritiated amino acids and purified by NP-40 lysis, affinity chromatography on columns of lentil lectin and specific immunoabsorbents, followed by SDS-PAGE. In some cases the immunoabsorbent chromatography was replaced by immunoprecipitation using SaCI. The radioactive polypeptide chains were eluted from the gel and characterized by peptide mapping and by sequence analysis of the N-terminal regions of the 25,000 MW chains. The preliminary results indicate that all three 25,000 MW chains studied were homologous.