A correlation between mu(HAc), (specific acetic acid accumulation rate) and mu (specific growth rate) for a recombinant Escherichia coil BL21 strain was defined under typical conditions to achieve high cell densities (fed-batch process, dissolved oxygen concentration higher than 30% saturation, semi-synthetic medium). The feeding rate of glucose was continuously adjusted in order to support constant values of mu (0.4, 0.3, 0.2, 0.1 and 0.05 h(-1)) during each of the five runs. An exponential correlation between mu and mu(HAc) showed significant increments in mu(HAc) for mu values higher than 0.30 h(-1). The metabolic activity of acetic acid synthesis was not suppressed, even at mu as low as 0.05 h(-1), a result which is not in agreement with previous published data. (C) 2000 Elsevier Science Ltd. All rights reserved.