The effects of hepatic triglyceride lipase (HTGL) and lipoprotein lipase (LPL) on the anti-Xa clotting activity of plasma were studied. LPL had no effect, but HTGL enhanced anti-Xa activity. This enhancement was shown to be due to a time-dependent action of HTGL on lipoproteins. These results could explain the increases in anti-Xa clotting activity previously observed after injection of heparin analogues, SSHA and SP54, which are potent releasers of lipase enzymes.