  We consider the implications of the recent determination of the universal
infrared background for the propagation of photons up to 20 TeV from the active
galaxy Markarian 501 as observed by HEGRA. At 20 TeV the mean free path for
photon-photon collisions on the infrared background would be much shorter than
the distance to Markarian 501, implying absorption factors of the order of
exp(-10), or greater, and consequently an excessive power output for this
active galaxy. Possible solutions of this problem are discussed.
