A radiochemical method for selective measurement of postheparin lipase activities was adapted to analyze lipoprotein lipase and hepatic lipase in preheparin plasma. The assay sensitivity was increased about four-fold by doubling both the volume of plasma used and the volume of lipolytic products taken for liquid scintillation counting, and was further improved by increasing the incubation period by 50% to 90 min. Rabbit antiserum to human hepatic lipase was unsuitable for the selective measurement of lipoprotein lipase because of apparent endogenous lipolytic activity. Preheparin hepatic lipase, however, was sensitive to inactivation by sodium dodecyl sulfate (SDS), the inhibition being greatest (> 90%) for plasma incubated with an equal volume of 40 mmol/L SDS. Intra- and interassay CVs for the two enzymes were 12.5-14.6% and 17.4-19.7%, respectively. In a cross-sectional study of 84 healthy subjects, pre- and postheparin hepatic lipase activities were higher in men than women, were correlated with indices of obesity, and were significantly correlated with one another, which explained the association of the former with plasma concentrations of high-density lipoprotein (HDL), HDL2, and small, dense low-density lipoproteins. There was no significant relationship between pre- and postheparin lipoprotein lipase activities, but the former were correlated with plasma concentrations of free fatty acids (FFA) and very-low-density lipoprotein. Apparently, preheparin activities of hepatic lipase, but not of lipoprotein lipase, may be a useful measure of the physiological function of "whole body" enzyme activity in cross-sectional and metabolic studies, where heparinization is not possible. Preheparin lipoprotein lipase activities, however, may reflect displacement of the enzyme by FFA and subsequent binding to remnants of triglyceride-rich lipoproteins.