[Zope] Multithreading in zope and python

I am using Zope-2.1.6. I have python 1.5.2 with multithreading on. But my zope still runs in single thread. I have changed the start script, to have PYTHONHOME="/usr:$reldir" and taken out the -D flag. Any suggestions on how to turn on multithreading will be appreciated. Thank you, -- sudhir