Hi Chuck. I have attempted to use the acetomethylester form of H2DCF-DA to
measure reactive oxygen species in endothelial cells. Molecular probes say that
this is the best probe for this type of work. Well, I had no joy what so ever.
As soon as I illuminated my cells with fluorescence to  locate them on the
cover slip the dye reacted  resulting in no fluorescent signal by the time you
were ready to start. Molecluar probes said that they had not heard of any
problems with the dye. This is because probably not many people have used it
yet! I am also keen to measure reactive oxygen species, but have not yet found
a suitable dye. Also it is important to remember that some of these dyes are
also sensitive to pH changes and therefore unless you are careful to measure
the pH throughout the experiment, alot of the time you may just be seeing a
reaction to a pH change. Good Luck SEAN