In a message dated 2/1/02 12:18:02 PM Central Standard Time, 
blproverbs@rediffmail.com writes:

<< histonet@pathology.swmed.edu >>
I am not familiar with the exact article being used as a reference, but skim 
milk by itself  will not effectively block all endogenous biotin.  It will 
clean up background due to endogenous +/- charges within the tissue.

Please understand that the avidin/biotin block is a two step process.  A ten 
to fifteen minute incubation in avidin, which binds to the endogenous biotin, 
and then a ten to fifteen minute incubation in biotin.  The incubation in 
biotin is necessary due to avidin having four binding sites for biotin.  
Theoretically the avidin will use one site to bind to endogenous biotin, 
leaving the other three available.  Without the biotin incubation these sites 
would bind to the biotin reporter molecule on the link or secondary antibody.

A further and separate blocking step is the use of  0.5% casein (Power Block, 
BioGenex) or 10% goat serum to block endogenous +/- charges on the tissue.  
Be aware that a very few antibodies can have diminished staining when using 
the casein, ER in particular.

Hope this helps.

Good Luck.