  We consider the effect of ambient matter on the Majorana phase of neutrinos.
We find that this can lead to an observable signal if a neutrino oscillation
experiment could be performed where the source and the detector are at
appropriately different matter densities. We illustrate the situation using a
beta beam neutrino source as an example and show that a 5 sigma signal for the
matter modification of the Majorana phase could be possible in a 5-year run.
