To evaluate phospholipase A activity a new assay is suggested. This assay is based on the recording of boundary potential changes of the planar bilayer lipid membrane during enzymatic hydrolysis of lipids. To register these changes, a second harmonic method is used. Sensitivity of the assay is about 0.0002 units/ml regardless of the impurities that may be present in the samples. One analysis takes about 5 min.