Abstract: A method for the determination of the amount of lipase in a sample comprises the steps of:(a) contacting the sample with a reagent composition comprising:(i) a lipase substrate which is a glycerol triester oil having in one of its two .alpha.-ester positions a long chain alkyl group having at least 8 carbon atoms and, in its two remaining ester positions, short chain alkyl groups such that, if the long chain alkyl group is hydrolyzed, the resulting diester is water soluble; and(ii) an esterase enzyme capable of catalyzing the hydrolysis of a water soluble glycerol diester to glycerol; and(b) detecting the rate at which glycerol is formed.The compositions of the present invention include the substrate and enzyme as defined. The element of the invention comprises a support having thereon the described composition. The invention is useful for determining lipase in samples which contain endogenous glycerol, such as blood serum and other body fluids.