> Would you know why does the DAB solution precipitate  when applied on the
> tissue.  I have been always using Vector DAB kit. This usually happens when
> the staining is done, so it doesn't  interfere with the results. I  wash my
> sections with PBS before applying the DAB solution.

  If you dissolve DAB directly in buffer (pH 7.2-7.6), you get
  an opalescent solution because the sparingly soluble free base
  is released and is reluctant to redissolve. To avoid this,
  dissolve the DAB hydrochloride first in  a small amount
  (about 1 ml) of distilled water, and then mix this solution
  with the larger (50 ml or whatever) volume of the buffer.
  If you do it that way there is never a precipitate.

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1