Abstract: A message data processing system suitable for processing messages identifying transactions includes an input processor, a parser, one or more repositories, and a data processor. The input processor receives data representing a message. The parser parses received message data to identify an individual data field in the message data. The one or more repositories contain ancillary information associated with data fields of the message data. The data processor retrieves ancillary information, associated with the identified data field of the message data, from the one or more repositories, and processes the retrieved associated ancillary information together with information in the data field for output.