An expert system, which included a database of rules and frame sets, was constructed for the purpose of improving the culture conditions in the cultivation of Escherichia coli. The inferring procedures were stored as production rules in the database, while data relating to media composition, the amounts of chemical elements in each component, and other culture conditions were stored as frames. In a primary experiment, the expert system suggested insufficiencies in the contents of chemical elements in the medium (modified LB medium) by comparing their amounts with the amounts of the elements in another medium used to achieve high growth. With the addition of medium components containing the amounts of chemical elements inferred to be insufficient, much higher growth of E. coli was obtained than in the cultivation without such addition. The system could also examine the total medium composition, which was equivalent to the sum of the compositions of the initial medium and the feeding solution, by comparing the contents of the chemical elements in it with those in the cell mass measured during the cultivation, and suggest the insufficiencies in these elements.