In response to the discussion on endogenous biotin, there is a method given in
the book,  Immunocytochemistry by Julia Polak and Susan Van Noorden, 2nd ed,
(1986), ISBN 0 7236 0870 9. 
The method is as follows: After your peroxidase block if any, rinse in PBS/TBS
as normal, apply Avidin at 1mg/ml for 20 minutes at room temperature, rinse as
above, then apply biotin at 0.1mg/ml for 20 minutes, rinse and proceed with
staining as normal.

This method works well and has the advantage that you know precise composition
and timings in the event of requiring GLP compliance, etc. The reagents are
available from Sigma.

Best wishes,

Margaret

Margaret Blount
Colworth House
Unilever Research
Beds
UK