The two steps in the biotin block procedure are both necessary for the block
to completely work. The first step involves a solution of avidin (either
bought or "home-brew" egg whites). This blocks the endogenous biotin in the
tissues. Now, this avidin needs to be blocked so as not to bind with your
detection system. The second step is a  solution of biotin (either bought or
skim milk), that binds with the avidin sites that are left open in the first
step. The added bonus of using skim milk is that you do get a bit of protein
blocking, too. Hope this helps clarify things.

Patti Loykasek
Phenopath Laboratories
Seattle, WA