Abstract: Phospho-vanillin reagent suitable for reaction with a mixture of sulfuric acid and an animal fluid containing at least 1 mg/ml lipids to determine total lipid concentration in the animal fluid comprising an aqueous solution containing vanillin and phosphoric acid having a vanillin to phosphoric acid mole ratio between 1.12 .times. 10.sup.-.sup.3 and 2.08 .times. 10.sup.-.sup.3. A sample of the animal fluid is mixed with concentrated sulfuric acid in a container, incubated, and the phospho-vanillin reagent is added to the mixture in the same container to form a chromogen. The light absorbance of the chromogen is measured and the total lipid concentration is determined in accordance with Beer's law from a standard curve relating total lipid concentration as a function of light absorbance by the chromogen.