Synthesis of sulphated proteoglycans by cell lines derived from explants of 7 rheumatoid and 9 normal specimens of synovial tissue, as well as by 7 lines of skin fibroblasts from non-rheumatoid patients, was examined. Cells of all 3 types were cultured as monolayers. They were then disaggregated and their capacity to synthesise proteoglycan estimated in cell suspensions by the incorporation of [35S]-sulphate into CPC-precipitable material during 2 hours of incubation. Cell suspensions incorporated somewhat more [35S]-sulphate than corresponding duplicate monolayers. Synovial cells from rheumatoid patients incorporated 2 to 3 times as much [35S]-sulphate as synovial cells from normals. Skin fibroblasts, however, incorporated less [35S]-sulphate than rheumatoid or normal synovial cells up to the fifth passage. Thereafter their incorporation gradually increased to overtake that of synovial cells. About one-half to one-third of the total [35S]-sulphate labelled material was closely associated with cells from synovial tissues and fibroblasts respectively.