SOME galactic clusters are strong X-ray emitters1, and we here the problem of establishing certain models for the source of this radiation. In the first model we suppose that the X rays are thermal bremsstrahlung from a hot quasi-isothermal inter-cluster gas. Typical parameters for the temperature, particle density and total mass of the gas are2 T ~ 3 × 108 K, n ~ 10−3, M ~ 3 × 1044 kg. If we assume3 relative nuclei solar abundances, then the thermal energy contained in the gas is E T ~ 2 × 1064 erg. This is ~10% of the total rest mass energy of a 1011 M circle dot galaxy and clearly poses a problem for potential energy sources.