Plalinax II, a cobalt-platinum alloy, is one of like most powerful permanent magnet materials knotvn. Because of its high performance, its principal applications are in miniaturised units where size and weight considerations are of the utmost importance, or where the geometry of the magnet excludes the use of other materials. It is readily fabricated and machined, enabling magnets of complex shape to be manufactured to close tolerances, and it will operate for long periods in highly corrosive environments. To obtain the maximum advantage from its use, it is necessary to design magnetic circuits carefully so as to exploit as fully as possible the magnetic characteristics of the alloy. Data are given in this article that will assist engineers to make the most effective use of Platinax II.