I use 2% paraformaldehyde 15 minutes at 4 degrees. It works great for
3T3 or COS cells transfected with GFP. Are you sure you get mature GFP
expressed? I had lots of problems initially. I had to drop the
incubation temp below 30 degrees for the protein to fold up properly. I
just started working with the "humanized" GFP and its MUCH better. I
can see some cells by 4 hours and LOTS by 16 hrs.
Hope this helps.