The presence of a lipase of hepatic origin in postheparin plasma of experimental animals has been confirmed by several investigators [1-6]. An enzyme with similar characteristics has also been found in humans [7-9]. This enzyme hydrolyzes triglycerides, phospholipids and acylCoA thiolesters [4, 9] and has been shown to be a transacylase [10]. Hepatic lipase clearly differs from lipoprotein lipase of extrahepatic origin in its co-factor, requirement and immunological reactivity. Also, it is not inhibited by high ionic strength in the assay medium [5, 7, 11, 12]. One of the characteristics of lipoprotein lipase is that it needs for full activity a VLDL-apoprotein, apoC-II. This has been demonstrated for purified lipoprotein lipase from adipose tissue [14], heart [15], postheparin plasma [4] and milk [16]. A recent study with bovine milk lipoprotein lipase indicates, that depending on the substrate concentration and the presence of apoC-lI in the assay medium, also apoC-I and apoC-III can stimulate fatty acid liberation [17]. We now report, that low concentrations of serum activate purified human postheparin plasma hepatic lipase. The C-apoproteins from VLDL did not cause this stimulation, but all of them suppressed the enzymatic activity.