A new bioassay for lipase utilizes a dim mutant of luminous bacteria which emit light upon the addition of long chain fatty acids, especially myristic acid. The luminescence response is proportional to the amount of added myristic acid over a 100-fold range, down to 10 nM. Trimyristin was used as a substrate for lipase and the hydrolyzed myristic acid was determined by the response of the luminous bacteria either on a continuous basis in the same reaction mixture or alternatively, when the hydrolytic stage is done separately followed by the independent detecting system. Using these procedures it is possible to assay lipase activity at rate corresponding to a release of as low as 10 pmol myristic acid per min.