3ds Max stores groups of materials in a library file with the .mat file extension. The API provides methods to access the materials stored in these libraries. You can use the methods Interface::LoadMaterialLib() and Interface::SaveMaterialLib(). This class also has a method GetMaterialLibrary() which returns a reference to the currently loaded library. You may then use class MtlBaseLib to access the materials themselves. The sample code below shows how this is done. It loads a materials library, removes a single material, and saves it again.
void TestMaterialLib() { // TODO: Change the path TCHAR *n1 = _T("D:\\3DSMAX\\MAPS\\3DSMAX.MAT"); int okay = ip->LoadMaterialLib(n1); if (! okay) return; // Declare this as references since we want to manipulate the // original material library and not a copy. MtlBaseLib &mlib1 = ip->GetMaterialLibrary(); intindex = mlib1.FindMtlByName(TSTR(_T("Aqua Glaze"))); if (index != -1) mlib1.Remove(mlib1[index]); ip->SaveMaterialLib(n1); }