I seem to remember that following treatment in Periodic Acid, Sodium
Borohydride can be used as an agent for blocking peroxidase.
I tried this years ago to see if it was less violent on cytology specimens -
don't think it was.

-- 
Lawrence Brett
Immunocytochemistry Lab.
Pathology Dept.
Western General Hospital
Crewe Road South
Edinburgh  EH4 2XU
Tel: 0131 537 2148
email: lauriebrett@bigfoot.com