High and low affinity heparin (HA and LA heparin) were prepared from commercial heparin by affinity chromatography to insolubilized antithrombin III. HA heparin was radiolabeled with 35S and subdivided by gel chromatography into high molecular weight (HMW, average 17,000– 26,000 daltons), intermediate molecular weight (MMW, average 12,000– 13,000 daltons), low molecular weight (LMW, average 5,000–7,000 daltons), and very low molecular weight (VLMW, average 4,600 daltons) fractions. The kinetics of lipolytic and anticoagulant activity and protein-bound radioactivity were studied after intravenous injection of these fractions. LA heparin failed to induce anticoagulant activity but released the hepatic triglyceride lipase (H-TGL) and lipoprotein lipase (LPL) activities normally. VLMW and LMW heparin failed to release both lipolytic enzymes and did not induce anticoagulant activity measurable by the activated partial thromboplastin time (APTT). A powerful anticoagulant effect was found in the anti-Xa assay, which disappeared according to a continuously concave curve in semilogarithmic plots, with elimination rates similar to those of the protein-bound radiolabel. The other heparin preparations induced all activities measured. Heparin anticoagulant activity estimated by the two assays disappeared following a convex curve, preceded by a rapid initial elimination phase in semilogarithmic plots. The disappearance rates of plasma protein-bound heparin radioactivity and heparin anticoagulant activity estimated by factor Xa inactivation were similar. Peak values of the two lipolytic activities were attained rapidly. H- TGL activity, as well as LPL activity, disappeared following convex curves in semilogarithmic plots, with elimination rates similar to those of plasma protein-bound heparin radioactivity. On the basis of these kinetics, we suggest that, after intravenous administration of heparin, the two lipolytic enzymes present in plasma are complexed with heparin, analogous to the heparin-antithrombin III complex. Finally, the kinetic data indicate that elimination of these activities is determined by the heparin part of the complexes, probably by removal of free heparin.