Bovine milk lipase was noncovalently bound to a heparin-Sepharose support and a [3H]glycerol/[14C]triolein emulsion was circulated through it. This system, more closely simulating in vivo conditions than the standard lipoprotein lipase assay, was employed to determine the effect of human apo-E and apo-C-II on the lipolysis of the circulating substrate. Both apo-C-II and apo-E produced enhanced lipolysis in comparison to unsupplemented emulsions, at appropriate enzyme densities on the heparin-Sepharose. With high enzyme densities the stimulation produced by apo-E was lost but that of apo-C-II persisted. When apo-E and apo-C-II were added together they produced significantly more lipolysis than when either was added separately. The enhancement of lipolysis produced by apo-E was correlated with the increased binding of triglyceride to the heparin-Sepharose enzyme complex. The effect of additions of both apoproteins to rat intestinal chylomicrons resulted in data quite similar to the triglyceride emulsions. Heparin-Sepharose columns with high and low zones of enzyme density produced greater lipolysis than when the enzyme was distributed more uniformly throughout the column. Perfusions of substrate supplemented with sufficient apo-E to produce maximal binding and lipolysis resulted in a progressive elution of the triglyceride substrate from the column during the perfusion. Free fatty acid:albumin molar ratios greater than 2 resulted in desorption of substrate from the column. This suggests the possibility of regulation of the lipolytic process by the products of lipolysis.