I have probably said this before but it bears repeating!  I think I
got caught out using TUNEL (Boeringer Mannheim kit - but it applies
to all).
Endogenous nucleases can be a source of non-specific staining (or
maybe just unwanted staining, but that's another debate).
Diethyl pyrocarbonate can inhibit the endogenous nuclease.
See:  Stahelin, Marti. Solioz, Zimmermann & Reichen.  J Cliinical
Pathology: Molecular Pathology.1998;51:204-208.
In my results, there was a lot of positivity and little correlation
with morphological indicators of apoptosis.
Russ Allison, Wales