1. A selective immunochemical method was used to measure post-heparin plasma lipoprotein lipase and hepatic lipase activity in eighty-two normal subjects and in twenty patients with type IIb, IV or V hypertriglyceridaemia. In twenty-six normal subjects the activity of post-heparin plasma lipases was compared with the kinetic parameters of endogenous plasma triglyceride metabolism.

2. The activity of post-heparin lipoprotein lipase was significantly higher in normal females than in males, whereas the activity of hepatic lipase showed an opposite sex ratio. The activity of lipoprotein lipase decreased with age both in males and females, whereas no significant age variation was observed in the activity of hepatic lipase.

3. In normal subjects a highly significant negative correlation was present in both sexes between the activity of post-heparin plasma lipoprotein lipase and fasting serum triglyceride concentration, but not between the activity of post-heparin hepatic lipase and serum triglycerides.

4. The fractional removal rate of endogenous triglycerides was positively correlated to the activity of lipoprotein lipase but not to the activity of hepatic lipase. No relationship was found between the activities of post-heparin plasma lipases and the absolute turnover of serum triglycerides.

5. The mean activity of post-heparin plasma lipoprotein lipase was significantly lower in subjects with hyperprebetalipoproteinaemia than in normal individuals. However, many hypertriglyceridaemic patients had lipoprotein lipase within the normal range and there was no correlation between serum triglyceride concentration and the activity of post-heparin lipases.

6. All three patients with fasting chylomicronaemia had low post-heparin lipoprotein lipase activity. Several subjects with high post-heparin plasma hepatic lipase activity were present in the group with hyperprebetalipoproteinaemia, but the mean value of the hepatic lipase was not significantly different from normal.

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