The fine epitope specificity of two murine monoclonal antibodies (HP 2G2 and HP 4D3)1 raised against rabbit xiphoid cartilage proteoglycan monomer (fraction AlDlDl) was determined by solid-phase radioimmune assay utilizing native and heat-denatured (50 degrees C, 30 min) AlDlDl as antigen. Competitive inhibition assays using trypsin-digested native AlDlDl fragments resolved by DEAE-cellulose chromatography and isopycnic CsCl density gradient ultracentrifugation showed that HP 2G2 reacted with a recurring epitope on the core protein, whereas HP 4D3 reacted with intact native and heat-denatured AlDlDl, but not tryptic peptides of AlDlDl. However, HP 4D3 was competitively inhibited from binding to native intact AlDlDl by the clostripain limit digestion products of AlDlDl partially purified by Sepharose CL-2B chromatography. HP 4D3 when added to rabbit but not human chondrocytes in culture affected the incorporation of 35SO4 into proteoglycans found in the most dense CsCl density gradient fraction (A1) under associative conditions.