  The energy spectrum of Mkn501 in the TeV energy regime, as measured by the
HEGRA (High Energy Gamma Ray Astronomy) Cerenkov telescopes during its low
state in 1995/96 and during a fraction of the 1997 outburst in the TeV energy
regime, is shown to place stringent upper limits on the still unknown infrared
photon density in the energy region between 3 x 10**(-3) and 3 xt 10**(-1) eV.
Assuming two different shapes for the unknown infrared photon spectrum in this
energy range we calculate upper limits on the infrared photon density on the
basis of the power-law fit obtained for the observed spectrum up to the maximum
energy.
