Patrick:

You can use a section of small intestine that contains argentaffin cells as a 
control for this stain.  Even though the stain is for argyrophil substances, 
it will also stain argentaffin substances, because they will automatically 
reduce the silver without a chemical reducer.  So to differentiate between 
argentaffin and argyrophil substances you need to run both the Gremilius and 
a Fontana-Masson or other argentaffin stain.  Then you need to compare.

Freida