Rendering without materials

How can I render my scene without any materials? Maybe there is sth like a special button or option?

In Render Buttons -> Render Layers you find the an input Field named “Mat:”. Here you can type in a name of a material which will override all other materials in the scene (during the rendering!), thus rendering all objects with the same material.
If you use the default material, it works as if there were no materials at all.