Three different assays for selective measurement of plasma lipoprotein lipase (LPL) and hepatic triglyceride lipase (H-TGL) were compared. These were: an immunochemical method based on enzyme antibody precipitation (IM), a procedure in which both enzymes were separated by affinity chromatography on small heparin--Sepharose columns (HS), and an assay in which one enzyme was inhibited by protamine sulfate (PS). Good correlations were found between the immunochemical and the heparin--Sepharose method, but not between these and the protamine sulfate assay procedure. The IM was then used to evaluate the effect of clofibrate on the two lipolytic enzymes. It was found that both in normals and in patients with Type IV hyperlipoproteinemia, clofibrate treatment leads to a specific increase of plasma LPL while H-TGL activity remains almost unaffected. The magnitude of the LPL response was different in normals and in patients with endogenous hyperlipoproteinemia. Furthermore, in normals the maximal increase of LPL activity was already reached one week after drug treatment was begun, while in hypertriglyceridemic patients, this effect was not evident prior to four weeks of clofibrate treatment. The marked enzyme increase following clofibrate administration indicates that an increased peripheral removal rate for triglycerides is one major mechanism responsible for the lipid-lowering effect of this drug.