The main lipoprotein density classes, namely very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), high-density lipoproteins2 (HDL2) and HDL3 were investigated with respect to their influence on hepatic lipase (HTGL) activity in vitro. Lipoproteins from pooled normal plasma (NP) and from pooled hyperlipemic plasma (HP) were prepared by means of sequential ultracentrifugation. Hepatic lipase was determined radioenzymatically after preincubation with protamine sulfate. It could be demonstrated that IDL from HP were able to stimulate HTGL activity by approximately 100% above the baseline value. HDL3 from both NP and HP revealed an inhibiting effect on HTGL activity. VLDL, LDL, and HDL2 exhibited no significant effect on HTGL activity. It is speculated that HTGL could possibly represent a second pathophysiological pathway for the catabolism of IDL in hyperlipemia but this presumption is supported by only a few investigations in vivo.