Cartilage glycosaminoglycans (GAGs) were measured by a spectrophotometric assay in synovial fluid obtained from 30 normal bovine hock joints and 15 osteoarthritic human knee joints. Results were compared with those obtained by radioimmunoassay (RIA). The spectrophotometric method (dimethylmethylene blue (DMB) assay) was found to be simple, safe, and sufficiently reproducible to be of potential value for serial measurement of sulphated GAGs in arthritic joint fluids. The nature of the proteoglycans present in normal bovine and osteoarthritic human synovial fluid was examined by gel chromatography. Whereas normal bovine synovial fluid contained only small molecular weight proteoglycans, osteoarthritic human synovial fluid contained aggregated proteoglycans and predominantly high molecular weight proteoglycan subunits.