  Following our previously proposed technique, we have used the recent
gamma-ray observations of Mrk 421 to place theoretically significant
constraints on and possible estimates of the intergalactic infrared radiation
field (IIRF) which are consistent with normal galactic IR production by stars
and dust and rule out exotic mechanisms proposed to produce a larger IIRF.
Using models for the low energy intergalactic photon spectrum from microwave to
UV energies, we calculate the opacity of inter- galactic space to gamma-rays as
a function of energy and redshift. These calculations indicate that the GeV
gamma-ray burst recently observed by the EGRET experiment on CGRO originates at
a redshift less than approximately 1.5.
