It doesn't really matter which one comes first - when I am using 
phospho-antibodies the IHC works better when the peroxidase block is=20
done _before_ antigen retrieval.  I have done the peroxidase block after 
antigen retrieval but before primary antibody, and also sometimes have 
waited until just before/after  the biotinylated secondary antibody. =20
The bottom line is that the peroxidase block needs to be done at some=20
point before using the horseradish peroxidase (HRP complex), so that any 
endogenous peroxidase can be quenched and not give you a lot of 
background (especially with any RBC in the tissue).

Rene J Buesa wrote:

>Antigen retrieval always comes first.
>René J.
>
>--- On Thu, 9/25/08, Monfils, Paul  wrote:
>
>From: Monfils, Paul 
>Subject: [Histonet] which one first - antigen retrieval or endo peroxidase block?
>To: histonet@lists.utsouthwestern.edu
>Date: Thursday, September 25, 2008, 2:26 PM
>
>When doing immunoperoxidase staining on a tissue that requires antigen
>retrieval, which do you do first - the antigen retrieval or the endogenous
>peroxidase blocking?  Or does it matter?
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