gr8place

PBS/Tween 20 CAN be used, in many cases, for blocking in ELISA! I have used this for many BSA-sensitive ELISA assays, eg. TGF beta, IGF's, etc...Tween 20 acts as a blocker itself.

I seem to remember that you can not boil casein, try warming it up to like 60C with stiring. PBS/tween20 is often used as blocking buffer for ELISA.