This paper investigates the kinetics and mechanism of loss of the two major proteoglycan species from cultures of bovine collateral ligament. Following incubation of ligament with [35S]sulfate after 6 days in culture, the rate of loss of the predominant proteoglycan species, decorin, from the matrix was shown to be much slower (t1/2 approximately 18 days) than that of the large chondroitin sulfate proteoglycan (t1/2 approximately 1.4 days). Analysis of 35S-labeled proteoglycans released into the medium between Days 11 and 15 of the culture period on a column of Sepharose CL-4B revealed that these macromolecules constituted mainly decorin of similar hydrodynamic size to that present in the matrix. Furthermore, analysis of core proteins using gel electrophoresis followed by fluorography or immunodetection with LF-94, an antibody directed against the amino-terminal region of decorin, indicated that the core proteins of decorin released into the medium and those remaining in the matrix of ligament cultures had a similar molecular mass (approximately 49 kDa). Analysis of both the 35S-labeled and endogenous macromolecules using 5/6/3-B-3, an antibody directed against terminal unsaturated chondroitin-6-sulfate disaccharides, revealed that three core proteins with molecular masses greater than approximately 200 kDa were present in the matrix. Four additional core proteins (range approximately 80-200 kDa) derived from the large proteoglycan were detected in the medium of ligament cultures. These findings indicate that, unlike decorin, the loss of the large chondroitin sulfate proteoglycan from the matrix of ligament cultures involved proteolytic cleavage of its core protein. No difference in the hydrodynamic size of the 35S-labeled glycosaminoglycan chains derived from either proteoglycan species remaining in the matrix or released into the medium of ligament cultures was observed.