In a series of healthy normolipaemic, adult subjects, plasma triglyceride turnover, heparin-releasable lipoprotein lipase (LPL), and plasma HDL cholesterol were determined. Strong correlations were found to exist between the fractional removal rate of plasma triglycerides (FTR) or LPL as one variable and HDL cholesterol as the other, whereas the triglyceride turnover rate (TR) did not correlate with HDL cholesterol. It is concluded that lipoprotein lipase action is largely responsible for the formation of HDL cholesterol in the blood stream.