Acidic proteins were isolated from synovial fluid from two osteoarthritic and two rheumatoid arthritic patients and identified by MS. It was found that the most abundant protein in all of the samples was the mucin-like protein lubricin. Further characterization of lubricin from the different patients by LC (liquid chromatography)-MS of released oligosaccharides showed that the core 1 O-linked oligosaccharides NeuAcα2-3Galβ1-3GalNAc and NeuAcα2-3Galβ1-3(NeuAcα2-6)-GalNAc were the dominating structures on lubricin. The latter was found to be more prevalent in the rheumatoid arthritis samples, indicating that sialylation is up-regulated as part of the inflammatory response. In addition to these dominating structures, core 2 structures were also found in low amounts, where the largest was the disialylated hexasaccharide corresponding to the sequence NeuAcα2- 3Galβ1-3(NeuAcα2-3Galβ1-3/4GlcNAcβ1-6)GalNAc. It was also found that a small proportion of the core 2 oligosaccharides carried sulfate. The ability of lubricin to present complex glycosylation reflecting the state of the joint tissue makes lubricin a candidate as a carrier of inflammatory oligosaccharide epitopes. In particular, it was shown that lubricin from inflamed arthritic tissue was recognized by the antibody MECA-79 and thus carried the sulfated epitope proposed to be part of the L-selectin ligand that is responsible for recruitment of leucocytes to inflammatory sites.