Try 0.002% aqueous neutral red for 5 mminutes, then
dehydrate, clear and mount in DPX or similar. It's
excited by near UV, blue or green light (325-546 nm).
The colour of the observed fluorescence ranges
from yellow to orange-red depending on the filter
system. Nuclei and cytoplasmic RNA are also shown.
It's a useful, cheap fluorescent stain.

If you use this dye at pH below 2 it _should_ become
selective for mast cells, cartilage matrix and
some types of mucus. I haven't tried it that way
myself; let us all know if it works!
-- 
-------------------------
John A. Kiernan
Department of Anatomy and Cell Biology
The University of Western Ontario
London,   Canada   N6A 5C1
   kiernan@uwo.ca
   http://publish.uwo.ca/~jkiernan/
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Nick Bullough wrote:
> I thought I'd check the collective wisdom of the group, one of my colleagues
> wants a fluorescent method for staining mast cells
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