Removing any traces of DNA

[Jean Peluso]

I would like to know how I can remove completely DNA from a preparation
of in vitro synthetised RNA. I have already tried the DNAse I recommended
by Invitrogen (T3 Transcription Kit), but when I perform RT-PCR on the
RNA 
preparation I unmask traces of DNA, even by using more DNAse and
prolonging
the incubation time of the reaction. The "highest" conditions I have tried
in order to degrade one microgram of DNA were: 30 min @ 37!C with three
units of DNAse. 

Any help would be welcome.
 
My e-mail address is: jean at esbs.u-strasbg.fr