The application of infrared absorption spectrophotometry the simulataneous determination of the predominant serum lipids is discussed. Quantification of the lipid extract from a 200 μl serum aliquot is based on the difference in the ester carbonyl absorption frequencies of triglycerides (νmax= 1742 cm−1), phospholipids (νmax= 1737 cm−1) cholesterol (νmax= 1723cm−1). Using a least-squares curve fitting technique in P-matrix notation, the optimum models for the Beer-Lambert Plots are C = P1A + POfor triglycerides and cholesterol and C = P2A2+ P1A + POfor phospholipids. Serum samples whose concentrations are outside of calibration limits (triglycerdes 30–209 mg/dl, phospholipids 72–385 mg/dl, and cholesterol 60–300 mg/dl) can be brought into range by appropriate dilution. Coefficients of variation (within-run) were less than 6% for all three components. Results of the infrared method are compared with standard clinical procedures.