More information needed,
normally a step in the process prior to this errror will give you a clue as to whats happening...check out the agent history for more information
see here for where else to look for this issue
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/replsql/replmon_3ek3.asp
make sure the subscriber is a valid subscriber and that the users you are using for it are in the PAL (publication Access list)
HTH
Adam
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Life is far too important to be taken seriously