Mesh Materials Problem

I''m having a problem when rendering meshes... Most of the meshes I''m using work ok, but on a few of them some materials completely ignore their place in life, and render themselves slap bang on top of everything else... Making the mesh look kinda funky... I''m using the plain old Tutorial mesh loading and rendering code (+ lighting) and i''ve checked the mesh out in the DX Mesh Viewer and it works ok in there... Any ideas? Cheers, Synex Code Monkey