Serum/plasma pairs of samples from 506 subjects were analyzed for cholesterol and triglycerides (triacylglycerols). In the 1 to 5 g/liter range for both cholesterol and triglycerides, values for serum averaged about 3% higher than the corresponding plasma values. We derived equations for the best-fitting lines relating the gram per liter concentrations of cholesterol and of triglycerides in serum and plasma: [Serum cholesterol] = 1.036[plasma cholesterol] -- 1.380[Serum triglyceride] = 1.031[plasma triglyceride] -- 0.37 No clinical significance is attribuled to the observed serum plasma differences, but they may be of importance when cholesterol and triglyceride values from different clinical trials or epidemiological studies are being compared.