Heparin-derived deca- and octa-saccharides were subjected to affinity chromatography on lipoprotein lipase-Sepharose and the fractions eluted at high salt concentration were analysed by strong-anion-exchange chromatography. Two high-affinity deca-saccharides were isolated and the structure determined by one- and two-dimensional ¹H-n.m.r. spectroscopy. The affinities of ³H-labelled low-molecular-mass heparin and size-fractionated deca-, octa-, and hexa-saccharides for lipoprotein lipase immobilized on microtitre plates were determined from saturation curves. From competition experiments the affinities of unlabelled heparins and pure deca- and hexa-saccharide fragments were determined. The binding was size- and charge-dependent, but structural dependency was also indicated. Thus substitution of a 2-O-sulphated L-iduronic acid with D-glucuronic acid was less important than the sulphation pattern of the D-glucosamine residue for affinity for lipoprotein lipase. Heparin inhibits binding of lipoprotein lipase to α₂-macroglobulin-receptor/low-density-lipoprotein receptor-related protein. The effects of size, charge and structure for this inhibition were studied. The ability of the heparin fragments to inhibit binding correlated with their affinity for lipoprotein lipase. This indicates that the inhibition of the binding of lipoprotein lipase to α₂-macroglobulin-receptor/low-density-lipoprotein receptor-related protein by heparin is exclusively mediated by binding of heparin to lipoprotein lipase.