Immunoreactive proteoglycans (iPGs) and sulphated glycosaminoglycans (GAGs) were assayed in synovial fluid obtained from 22 patients with osteoarthritis (OA), 21 with rheumatoid arthritis (RA), 13 with gout, and five with Reiter's syndrome. A strong positive linear correlation was observed between concentrations of sulphated GAGs and iPGs in RA (r = 0.95) and gout (r = 0.94). A linear correlation was also observed in OA (r = 0.65). Patients with gout and Reiter's syndrome had significantly higher concentrations of sulphated GAGs and iPGs than patients with OA or RA. Patients with gout also had significantly higher total quantities of sulphated GAGs and iPGs in the knee joint cavity than patients with OA or RA. In all four diseases similar profiles were observed when comparisons were made between the total quantities and concentrations of sulphated GAGs and iPGs in synovial fluid. These results indicate that the observed differences in concentrations are not simply a function of dilution. The concentrations of sulphated GAGs and iPGs did not correlate closely with the type or number of inflammatory cells in the synovial fluid. Considerable variation was noted in the sulphated GAG/iPG ratios, suggesting that different mechanisms may be contributing to the release of proteoglycans in the diseases studied.