Monoclonal antibodies to hyaluronidase-treated chondroitin sulfate proteoglycan (CSPG) were used to study the immunological determinants of chick cartilage proteoglycan. The determinants recognized by the antibodies were studied by a radioimmune inhibition assay utilizing hyaluronidase-treated [35S]CSPG. Hyaluronidase-treated CSPG inhibits the reaction of four clonal antibodies, S54C, S103L, S11D, and P100D, with [35S]CSPG, but to varying degrees. Only the reaction of S103L is inhibited to a considerable extent by undigested CSPG, indicating that hyaluronidase treatment exposes determinants specific for the other three antibodies. These findings are consistent with the earlier conclusion that S103L is specific for a protein determinant (Dorfman et al., 1980). Only the reaction of S54C is not significantly inhibited by chondroitinase ABC-digested CSPG. This result indicates that chondroitinase ABC digestion can also expose determinants recognized by S11D and P100D but that such digestion removes the determinant recognized by S54C. Of the four antibodies tested, only the reaction of S54C with hyaluronidase-treated [35S]CSPG is significantly inhibited by chondroitin-6-SO4 tetra- and hexasaccharide (59 and 43% inhibition, respectively, at a concentration of 1333 microM). The reaction of S54C is inhibited to a lesser extent by chondroitin tetra- and hexasaccharide (28 and 26% inhibition, respectively, at a concentration of 1333 microM). In contrast, chondroitin-4-SO4 oligosaccharides do not inhibit the reactions of any of the clonal antibodies. These result suggest that S54C recognizes a determinant that contains chondroitin-6-SO4 oligosaccharide, attached via the linkage oligosaccharide to core protein.