The glutamate dehydrogenase deamination assay based on the increase of absorbance at 340 nm, caused by the transformation of NAD into NADH in the presence of L-giutamate applied to crude plant extracts, is unreliable without dialysis of the extract to eliminate disturbing reactions. A low molecular weight substance, probably a phenolic compound or aromatic amine, was isolated from Petunia leaves which could act as a substrate or activator of another dehydrogenase. This dehydrogenase which interferes with the glutamate dehydrogenase deamination assay may use many amino acids and amines as substrate. The use of a 1 % concentration of insoluble polyvinylpyrrolidone in the extraction buffer causes a higher glutamate dehydrogenase activity for both amination and deamination. The glutamate dehydrogenase is partially present in a latent form.