Many extracellular proteins bind avidly to heparin in vitro, reflecting their affinity, in a physiological setting, for the long polysaccharide side chains of heparan sulfate proteoglycans (HSPGs). In most cases, the physiological significance of this interaction is not clear. Lutz and colleagues previously identified a putative heparin binding site in the secreted enzyme lipoprotein lipase (LpL), and they have now generated a transgenic mouse line expressing a version of human LpL that is mutated at this short sequence. The mutant protein binds poorly to heparin and loses activity rapidly when expressed either in cultured cells or in mice. Native LpL is active as a homodimer, but the mutant form readily dissociates into inactive monomers, suggesting that HSPGs help maintain the quaternary structure of the protein. The mutant LpL also fails to localize properly: Wild-type LpL remains firmly attached to the lumen of capillaries and releases lipoprotein-derived lipids at this site, whereas the mutant protein is only transiently associated with the capillary bed and is rapidly shed into the bloodstream. Not all of the released protein is inactive, so lipoprotein metabolism begins in the bloodstream of transgenic mice. The generation of fatty acids within the circulation, rather than in the muscle and other tissues, profoundly alter the blood lipid profile of LpL transgenic mice, even those that also carry the normal gene and express the native enzyme at its usual location.