An alternative form of the human invariant chain exists as a chondroitin sulfate proteoglycan (CSPG) with invariant chain as the core protein. The selective inhibitor of proteoglycan synthesis, p-nitrophenyl beta-D-xyloside was used to study the role of this CSPG in class II biology. At xyloside concentrations of 2.5 and 5.0 mM, CSPG synthesis was completely inhibited with marginal inhibition of protein synthesis. The inhibitory effect on CSPG synthesis was completely reversible. The number of class II molecules on the cell surface was not affected by xyloside, but biosynthesis and appearance of newly synthesized class II molecules at the cell surface were both decreased by xyloside. Recognition of influenza virus-infected cells by class II-restricted, virus-specific cytotoxic T lymphocytes was not diminished by the presence of xyloside in the effector phase of the cytotoxicity assay. However, sensitization of target cells was markedly inhibited when target cells were exposed to virus in the presence of xyloside. These results are consistent with the hypothesis that the CSPG form of invariant chain has a role in antigen processing.