JMS messages use the self-describing XML format and in particular the MQSeries format RFH2. However, it is possible to force MQ to send messages in the 'traditional', unformatted manner.

On the Destination (i.e. the Queue or Topic) use method:
  setTargetClient(type)

where type is:
 JMSC.MQJMS_CLIENT_NONJMS_MQ for 'traditional' MQ messages or
 JMSC.MQJMS_CLIENT_JMS-COMPLIANT means JMS messages

Note that this should work either within the program or for objects defined via JNDI.