The sulfated glycosaminoglycans, heparan sulfate and heparin, are increasingly implicated in cell-biological processes such as cytokine action, cell adhesion, and regulation of enzymic catalysis. These activities generally depend on interactions of the polysaccharides with proteins, mediated by distinct saccharide sequences, and expressed at various levels of specificity, selectivity, and molecular organization. The formation of heparin/ heparan sulfate in the cell requires an elaborate biosynthetic machinery, that is conceived in terms of a novel model of glycosaminoglycan assembly and processive modification. Recent advances in the identification and molecular analysis of the enzymes and other proteins involved in the biosynthesis provide novel tools to study the regulation of the process, presently poorly understood, at the subcellular and cellular levels. The potential medical importance of heparin-related compounds is likely to promote the biotechnological exploitation of components of the biosynthet...