As many of the replies indicate, the problem could be due to endogenous
biotin as well as endogenous peroxidase.  We've found that using one of the
available avidin-biotin blocking kits will help reduce the background.  You
could also change your detection from a standard ABC or LSAB to the Envision
technique which avoid the use of biotin.  We've used this kit with a few
antibodies on liver sections with good success.

Good luck,


Tim Coskran
Pfizer