The murine Ia-associated chondroitin sulfate proteoglycan (CSPG) was studied both biochemically and immunochemically to determine the nature of its core protein. Chondroitinase ABC or chondroitinase AC treatment of the CSPG digested the chondroitin sulfate glycosaminoglycan, yielding a core protein that migrated with an apparent molecular weight of 38,000. Comparative V8 protease digestion of the CSPG core protein and conventional invariant glycoproteins yielded homologous peptides, indicating that the core protein and invariant chain were structurally similar. The purified CSPG and its core protein were both shown to react directly with the monoclonal anti-invariant chain antibody, In-1. Comparative tryptic peptide analysis by high performance liquid chromatography demonstrated coelution of the majority of the peptides from the invariant chain and the CSPG core protein. Collectively, these results indicate that the CSPG is an alternatively processed form of invariant chain.