Staphylococcu8 produces at least three leucocidins, of which that originally described by Panton & Valentine (1932) is of particular interest since it attacks only white blood cells and not, like the other staphylococcal leucocidins, red blood cells as well. It is also specific to humans and rabbits. Some of the biological properties of this substance have been described by Gladstone & van Heyningen (1957). In the present paper the production of culture filtrates with high leucocidin activity is described and it is shown that this activity depends upon at least two proteins. These have been highly purified.