The active galaxy Markarian 501 was discovered with air-Cerenkov telescopes at photon energies of 10 tera-electron volts, Such high energies may indicate that the gamma rays from Markarian 501 are due to the acceleration of protons rather than electrons, Furthermore, the observed absence of gamma ray attenuation due to electron-positron pair production in collisions with cosmic infrared photons implies a limit of 2 to 4 nanowatts per square meter per steradian for the energy flux of an extragalactic infrared radiation background at a wavelength of 25 micrometers, This limit provides important clues about the epoch of galaxy formation.