In this report, we describe a previously unidentified component in the murine Ia antigen complex. SDS-PAGE analysis of anti-Ia immunoprecipitates prepared from spleen cells biosynthetically labeled with 35S-sulfate showed no detectable incorporation of 35SO4 into alpha, beta, or Ii chains but did not reveal the presence of a novel sulfate-bearing molecule of considerable molecular weight heterogeneity (46-69-kdaltons). The 46-69-kdalton molecule could be precipitated with monoclonal antibodies specific for I-A, I-E, and Ii glycoproteins but was not seen in control precipitates, nor in association with IgG or class I MHC molecules. Preliminary biochemical characterization indicated that the 46-69-kdalton product is extremely polydisperse, both in charge and apparent molecular weight, is sensitive to proteases, and bears the sulfate moiety on a large pronase-resistant structure. These results suggested this component might be a proteoglycan. Definitive identification of this component as a proteoglycan was accomplished by selective enzymatic degradation experiments which showed that the sulfate-bearing component of the 46-69-kdalton molecule is chondroitin 6-sulfate.