We have previously reported that liver sinusoidal endothelial cells (LSECs) are responsible for the clearance of monocyte chondroitin sulfate proteoglycan serglycin from the circulation (Øynebråten et al.(2000) J. Leukocyte Biol. 67; 183-188). The aim of the present study was to investigate the kinetics of degradation of endocytosed serglycin in primary cultures of LSECs. The final degradation products of serglycin labelled biosynthetically in the glycosaminoglycan (GAG) chains with [3H] in the acetyl groups of N-acetyl galactosamine residues, [14C] in the pyranose rings, or [35S] in the sulfate groups were identified as[3H]-acetate, [14C]-lactate and [35S]-sulfate. Comparison of the rate of release of degradation products from the cells after endocytosis of serglycin labelled chemically with 125I in the tyrosine residues, or biosynthetically with [35S] or [3H] in the sulfate or acetyl groups, respectively, showed that 125I appeared more rapidly in the medium than [35S]-sulfate and [3H]-acetate. Judging from the speed of appearance of free 125I both intracellularly and in the medium, the core protein is degraded considerably more rapidly than the GAG chains. Desulfation of the GAG chains starts after the GAG chains are released from the core protein. Generation of lactate and acetate as the final products from degradation of the carbon skeleton of the GAG chains indicates that catabolism of endocytosed macromolecules in LSECs proceeds anaerobically.