ALTHOUGH the existence of the cosmic X-ray background has been known for the past ten years, the origin of this radiation has not been identified. Many theories have been proposed (see reviews by Silk1 and Setti and Rees2), but many of these have more parameters than the observations could define and much use has been made of the so-called “spectral break” at about 40 keV. More recently, doubt has been cast on the sharpness and indeed existence, of the break.