RTF-Field only at Design-Preview OK, not when starting from .net Appl.

I am using CR 2008 and .net 2008.

At Design-Time, an RTF-Text field is displayd properly. Field is formatted with Text-Interpretation "RTF"

When I start this report - on the same machine (Windows Vista) like at design-time - from my .net-Application the Text is displayed with the formatting tags of an RTF-Field - like ignoring the text-interpretation "RTF".

Can anyone explain this and help me?