Dear moknows,
There is no command in AutoCAD to convert a 3D mesh to solids, but if you access the Mechanical Desktop, you can convert the mest to surfaces using the AM2SF command, convert the resulting surfaces to a part using the AMSTITCH command and at last convert the part to 3D solid using the EXPLODE command.
Farzad
Thank you for your responses. My main intention was to turn a drawing full of 'closed' poly meshes into solids so that the file size would not be quit as large. Currently these drawings are 40mb+.
I do not have Mechanical desktop, and the m2s lisp is a handy little tool, but appears to only work on 'open' meshes. So I have not been able to make it work yet. Your ideas have been great. Thank you.