If you analyze within 2 hours of fixation with 0.5% PFA, then you can use PI:
do standard PI staining, wash, then fix. If you wait more than 2 hours, the
signal really degrades.
The other alternative (which we have used for combining with intracellular
staining) is to use EMA (ethdium monoazide). Stain cells with 5 ug/ml in the
dark (20 min), wash twice, then expose to a UV source (like a UV light, even I
think, a fluorescent light bulb). Wash. EMA is UV-crosslinked to become
covalently bound to the cells and lights up cells that were dead before fixation
and permeabilization.
The EMA was published by a couple of people; see the Molecular Probes catalog
for specific references.
mr