I have a RH 9.0 server with j2sdk-1_4_2 installed.
I get this error when trying to install jython-21.
 
 java jython-21
Exception in thread "main" java.lang.NoClassDefFoundError: jython-21
   at 0x40265e17: java.lang.Throwable.Throwable(java.lang.String) (/usr/lib/./libgcj.so.3)
   at 0x40258c8e: java.lang.Error.Error(java.lang.String) (/usr/lib/./libgcj.so.3)
   at 0x4025a6b6: java.lang.LinkageError.LinkageError(java.lang.String) (/usr/lib/./libgcj.so.3)
   at 0x4025bb36: java.lang.NoClassDefFoundError.NoClassDefFoundError(java.lang.String) (/usr/lib/./libgcj.so.3)
   at 0x402aa075: gnu.gcj.runtime.FirstThread.run() (/usr/lib/./libgcj.so.3)
   at 0x4024cc4c: _Jv_ThreadRun(java.lang.Thread) (/usr/lib/./libgcj.so.3)
   at 0x402198ac: _Jv_RunMain(java.lang.Class, byte const, int, byte const, boolean) (/usr/lib/./libgcj.so.3)
   at 0x08048910: ?? (??:0)
   at 0x406945cd: __libc_start_main (/lib/libc.so.6)
   at 0x080486c1: ?? (??:0)
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2780547 Jan  6  2002 jython-21.class
34653233 Jun 20 04:53 j2sdk-1_4_2-linux-i586.rpm
 
If you need any other info, I will happily give it to you...