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To all who responded,  

Thank you so much for your advice.  A few summaries, comments, and a 
question..

1.  The suggestion of spectral unmixing is a very powerful one, but 
it would require that the "true" green signal be of sufficient 
strength that it would show up above the noise level after unmixing.  
Since the DsRed signal is remarkably strong, this does limit the 
approach.  It might be useful in future experiments.

2.  Several comments referred me to Baird's paper (PNAS 97:11984-- 
(2000)), which refers to the conversion of green to red over a period 
of two days.  Baird reports that the conversion occurs in pure 
solution, so it doesn't appear to require biological activity.  In an 
indirect way, I have confirmed this.  I looked at some slides that 
were about two months old, and there was no green component.

My question, then.  Does any of you know whether the conversion might 
occur in fixed (2% paraform....) tissue?  That would explain the 
results I mentioned above.

Again, thanks for all of your suggestions.

Joel

Joel B. Sheffield, Ph.D.
Biology Department, Temple University
1900 North 12th Street
Philadelphia, PA 19122
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(215) 204 8839, fax (215) 204 0486
http://astro.temple.edu/~jbs