This paper shows that differences in growth behavior of Escherichia coli strain HB101 and strain HB101[pGEc47] can be related to yeast extract-enriched medium rather than plasmid properties. An optimal medium for growth of E. coli HB101[pGEc47] was designed based on the individual yield coefficients for specific medium components (NH4+ 6 g g(-1), PO43- 14 g g(-1), SO42- 50 g g(-1)). The yield coefficient for L-leucine depends on the glucose content of the medium (20 g g(-1) for 3% glucose, 40 g g(-1) for 1% glucose) and the yield coefficient for L-proline depends on the cultivation mode (20 g g(-1) for batch cultivation, 44 g g(-1) for continuous cultivation). Growth on defined medium after medium optimization is as rapid as on complex medium (0.42-0.45 h(-1)). The critical dilution rate (D-R) in the defined medium above which undesired production of acetic acid occurs is in the range of 0.23-0.26 h(-1).