  We propose a test for non-standard neutrino-neutrino interactions by using
ultrahigh energy AGN neutrinos. Such interactions would influence the AGN
neutrino flux due to collisions with cosmic background neutrinos. For typical
AGN neutrinos we obtain an upper limit for the coupling constant $g<6.4\cdot
10^{-3}$ if the mediator is light and $g/(M_X/MeV) <0.013$ if the mediator is
heavy. We compare our results with constraints from other phenomena previously
considered.
