A new type of fluorogenic and isomerically pure 1(3)-O-alkyl-2,3 (3,2)-diacyl glycerols was synthesized that can be used as substrate for the determination of lipase activities. These compounds contain a fluorescent pyrene acyl chain and, as a potent quencher of pyrene fluorescence, a trinitrophenylamino acyl residue. In their intact form, the fluorogens show only low fluorescence intensity. Upon lipase-induced or chemical hydrolysis of the substrates, however, the fluorophore and quencher separate from each other. This leads to a gradual increase in pyrene fluorescence, reflecting the time-dependent progress of lipolysis and, under substrate saturation conditions, lipase activity. This lipase assay is continuous and does not require separation of substrate and reaction products. Short- and long-chain homologues as well as optical isomers of the fluorogenic alkyldiacyl glycerols were hydrolyzed by pancreatic lipase, hepatic lipase, and lipo-protein lipase at highly different rates depending on the substrate or enzyme preparation and source (e.g., postheparin plasma or cultured cells). It is proposed that a useful set of enantiomeric and/or homologous substrates in combination with appropriate reaction media might be applied to the selective determination of a lipase in a mixture of lipases, e.g., hepatic and lipoprotein lipase in PHP, for medical diagnostics.