DAPI is 4",6-diamino-2-phenylindole.

A good article for further review and handling, fixation

DAPI as a useful stain for nuclear quantitation.  Tarnowski, BI et al,
Biotechnic and Histochem, 66:296 302, 1991.  

Applications of DAPI cytochemistry to neuurobiology, Sanna PP et al, 
Biotechnic and Histochem 67:346-350, 1992  This was a very elegant 
procedure on tissue sections.

Had a postdoc asking the same question as you recently, he fixed with
acetone vs the usual formalin or PFA, and aquamount media.  
Worked very nicely.  Also had some biofilm 
gurus using this for bacteria in biofilm, without fixing the cells first.
They did air dry overnight before looking at the slides, and I don't think
they used mounting media with this method.  The did both fixed and unfixed
biofilm frozen sections, stained with DAPI.  
I would be very careful in handling this stuff with gloves on, if it stains
the DNA in other tissues so nicely, it will be lighting up your DNA also.

Gayle Callis