Mesh Animation - Ways to rotate part of a mesh?

Hi

I had a look through the code archives but I couldn't find anything to help me. I have a single mesh divided into 100 units, some of those units may be facing a diferent direction to others within that mesh. Are there any readily available functions or formulas to rotate part of a mesh around it's 'local' origin? All the units share the same 'local' origin.

Thanks for any help.

Cheers

GrumpyOldMan