Lipoprotein lipase (LPL) activity and mass in plasma and their increase after heparin administration were measured in 61 men who had suffered myocardial infarction before the age of 45 years and in 69 population-based age- and sex-matched control subjects without coronary heart disease to study the relations between these parameters in plasma and their correlations with plasma lipoproteins in subjects with a wide range of lipoprotein and LPL levels. There was a relatively large amount of LPL protein compared with LPL activity in preheparin plasma, indicating that the majority of circulating LPL is catalytically inactive. LPL mass and activity in postheparin plasma (postheparin minus preheparin values) were highly correlated, and the calculated mean specific activity (0.35 mU/ng) was in the range expected for catalytically active LPL. Hence, heparin releases mainly active LPL. The four LPL parameters (mass and activity in plasma and their increase after heparin administration) were not related to each other, except for postheparin plasma LPL mass and activity, and they showed different correlations with plasma lipoprotein lipid concentrations. There was a strong positive correlation between LPL mass in preheparin plasma and the HDL cholesterol level as well as weak negative relations to VLDL triglyceride and cholesterol concentrations in the patients. In contrast, preheparin LPL activity showed no correlation with the HDL cholesterol level but weak positive relations to VLDL triglyceride and cholesterol concentrations in the control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)