  Active galactic nuclei (AGN) have been suggested to be sources of very high
energy neutrinos. We consider the possibility of using AGN neutrinos to test
neutrino mixings. From the atmospheric, solar and laboratory data on neutrino
oscillations we derive the flavour composition of the AGN neutrino flux in
different neutrino mixing schemes. We show that most of the schemes considered
can be distinguished from each other and the existence of a sterile neutrino
can be specially tested. AGN neutrinos can also be used to test those
four-neutrino scenarios where solar neutrinos oscillate into an arbitrary
mixture of $\nu_s$ and $\nu_\tau$.
