Synovial capsule from the metacarpophalangeal joints of cattle was shown to be a highly collagenous tissue, with a hydroxyproline content of 100 +/- 1 micrograms/mg dry weight and a water content of 70 +/- 3.6%. Type-I collagen made up 83% of the collagen present, and the remainder was type III. When incubated in explant culture, synovial capsule incorporated [3H]acetate into both glycoproteins and hyaluronan and [3H]acetate and [35S]sulfate into proteoglycans. The rate of synthesis of proteoglycans by synovial tissue was shown to be similar to that measured for collateral ligament from the same joint. Two populations of proteoglycans were observed to be synthesized by synovial capsule. More than 90% of the 35S-labelled proteoglycans eluted with a K(av) of 0.7 on Sepharose CL-4B, and the remainder of the radiolabelled macromolecules eluted from the column with a K(av) of less than 0.5. Analysis of the major population of proteoglycans showed it to consist of a dermatan sulfate-containing proteoglycan with a core protein of 45,000 Da that had the same N-terminal amino acid sequence as decorin.