CacheItemUpdateCallback and dot net framework

Hi,

I introduced usage of CacheItemUpdateCallback in my ASP dot net application. The initial version of framework was 2.0.
I updated to 3.5 since CacheItemUpdateCallback was not supported in earlier versions.
On the development environment everything works fine.
On the test environment, I have initially dot net framework 2.0.
Now when I installed dot net framework 3.5, I get the below error while adding item to the cache and refering the callback.
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Could not load type 'System.Web.Caching.CacheItemUpdateCallback' from assembly 'System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
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I really appreciate any help I can get on this.

Thanks in Advance
Ajith
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