  Three independent predictions follow from postulating the existence of
protons co-accelerated with electrons in extragalactic jets (i) multi-TeV gamma
ray emission from nearby blazars, (ii) extragalactic cosmic ray protons up to
10^20 eV, and (iii) extragalactic neutrinos up to 5 10^18 eV. Recent gamma ray
observations of Mrk 421 and Mrk 501 employing the air-Cerenkov technique are
consistent with the predicted gamma ray spectrum, if one corrects for pair
attenuation on the infrared background. Prediction (ii) is consistent with
cosmic ray data, if one requires that jets are responsible for at least a
sizable fraction of the extragalactic gamma ray background. With kubic
kilometer neutrino telescopes, it will be possible to test (iii), although the
muon event rates are rather low. Neutrino oscillations can increase the event
rate by inducing tau-cascades removing the Earth shadowing effect.
