LISA should be able to detect the gravitational waves from the QNM ringdown of supermassive black holes in the 10⁵ – 10⁸ solar mass range. On the other hand, it is reasonable to think that any quantum theory of gravitation should impose the quantization of the energy levels of these QNM. Here we discuss the possibility of distinguishing quantum aspects of gravity using LISA to observe QNM overtones of highly excited supermassive black holes.PACS numbers: 04.80.Nn, 95.55.Ym