Field Calculate in Python Script

Hi,

I am trying to use the field calculator in a stand alone python script. I am trying:

expression = '(!"infield"!)*2'
arcpy.CalculateField_management(inshape,infield,expression,"PYTHON").

I am getting this error: ExecuteError: ERROR 000539: Invalid field "infield".

I think the syntax is right... any thoughts are greatly appreciated.

Thanks!