It was found that lipolytic activity in bovine post-heparin plasma differed from that of other mammalian species by the fact that intravenous heparin induced the release of lipoprotein lipase but not hepatic triacylglycerol lipase. Initially, this fact was strongly suspected when no remaining lipolytic activity could be found after whole bovine post-heparin plasma had been tested with either 1 M NaCl or antiserum against lipoprotein lipase. This was further confirmed by using heparin-Sepharose affinity chromatography when the entire lipolytic activity was eluted with 1.5 M NaCl but none with 0.4 or 0.7 M NaCl. The active fraction had lipoprotein lipase characteristics, i.e. it required serum activators to produce optimum activity and was fully inhibited by NaCl of high molarity and by anti-lipoprotein lipase antiserum. Neither the different doses of heparin nor the various times of sampling altered the results. This raises the question whether hepatic triacylglycerol lipase is absent from the bovine liver or whether this enzyme is present but cannot be released by heparin.