An assay for the determination of triglyceride (acyl glycerol) in microliter volumes of serum or plasma is described. The method is based on enzymatic hydrolysis of triglyceride by lipase from Rhizopus arrhizus followed by enzymatic fluorimetric assay of the glycerol so released. Under the conditions of the assay the enzymatic hydrolysis is complete for concentrations up to 5 mmol/l in less than 15 min at room temperature. The determination of free glycerol and glycerol released is based on the formation of dihydroxyacetone in the presence of NAD and glycerol dehydrogenase. The NADH which is formed in stoichiometric quantities is estimated fluorimetrically. In the presence of the ketone-trapping agent hydrazine the reaction goes to completion above pH 9.0. By this method acyl glycerol can be measured routinely in a 10 microliters sample of serum or plasma. The test is extremely sensitive compared to previously described methods and exhibits acceptable precision and reproducibility.