High-cell-density cultivations of Escherichia coli K12 in a dialysis reactor with controlled levels of dissolved oxygen were carried out with different carbon sources : glucose and glycerol. Extremely high cell concentrations of 190 g/l and 180 g/l dry cell weight were obtained in glucose medium and in glycerol medium respectively. Different behaviour was observed in the formation of acetic acid in these cultivations. In glucose medium, acetic acid was formed during the earlier phase of cultivation. However, in glycerol medium, acetic acid formation started later and was particularly rapid at the end of the cultivation. In order to estimate the influence of acetic acid during these high-cell-density cultivations, the inhibitory effect of acetic acid on cell growth was investigated under different culture conditions. It was found that the inhibition of cell growth by acetic acid in the fermenter was much less than that in a shaker culture. On the basis of the results obtained in these investigations of the inhibitory effect of acetic acid, and the mathematical predictions of cell growth in a dialysis reactor, the influence of acetic acid on high-cell-density cultivation is discussed.