DROP TABLESPACE error

Hello all. I'm a neophyte oracle DBA, and I was wondering if anyone could give me some hints/clues/suggestions as to what could be causing the following error when I try to use DROP TABLESPACE to drop a tablespace I wish to recreate (the name of the tablespace has been changed to protect the innocent :) -

SQL> DROP TABLESPACE xxxx INCLUDING CONTENTS; *
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1 ORA-02273: this
 unique/primary key is referenced by some foreign keys

I did take the tablespace offline before trying to drop it, and I did create a test tablespace with a couple of test tables which I SUCCESSFULLY dropped with the INCLUDING CONTENTS option. I've investigated a few other ideas and poked through a few FM's, but so far to no avail. I'm still plugging at it, but thought I might be able to pick the collective brain to save a lot of shooting in the dark.

Any replies either here or by email will be greatly appreciated.