A keratan sulfate proteoglycan was isolated under denaturing conditions from the mineral compartment of rabbit cortical bone. This small proteoglycan (Kd = 0.39 on Superose 6, Mr approximately 20,000 on sodium dodecyl sulfate gels) contained small keratan sulfate chains that were distinctly bimodal in size. The keratanase and endo-beta-galactosidase digestible glycosaminoglycan chains were O-linked to a core protein of Mr approximately 80,000. This core protein had several properties in common with the bone sialoprotein II molecule of bovine and human bone including: a closely spaced doublet band on sodium dodecyl sulfate electrophoresis gels; a high staining intensity with Stains All that was greatly diminished by neuraminidase; a significant amount of small O-linked oligosaccharides; and an amino-terminal amino acid sequence that was nearly identical to human bone sialoprotein II. (In contrast, bone sialoprotein II in human, bovine, and rat bone does not appear to have any keratan sulfate chains.) Antiserum made against the keratan sulfate proteoglycan reacted with its core protein on electrotransfers from sodium dodecyl sulfate-polyacrylamide gels.