Expert systems composed of a database and a knowledge base, which cooperate, are considered. In such systems mutual consistency of the database and the knowledge base must be enforced. A consistency monitor is proposed for this purpose, whose task is double: to enforce internal consistency of the database and mutual consistency of the database and the knowledge base. The first task comprise enforcement of referential constraints imposed on the database. An efficient technique is proposed. Referential constraints are enforced by the use of DBMS journals instead of looking up the entire database. The consistency monitor described in this paper is applied in the laboratory information and management system ELAN installed in the analytical laboratories of the nuclear fuel reprocessing pilot plan at the Nuclear Center of Karlsruhe.