In monocultures of micro-organisms, growth is controlled by feedback mechanisms involving chemical factors such as limiting substrates and inhibitory metabolic products. The role of such feedback in the growth regulation of Escherichia coli O-124 was investigated by growing cells in batch culture using a medium containing glucose and mineral salts. In various phases of growth, portions of the native culture were diluted with culture filtrate, so that although cell density decreased, the chemical composition of the growth medium was unaltered. As the diluted cultures grew, variations in growth acceleration were calculated and compared with those of native (undiluted) cultures. Towards the end of the exponential phase and in the growth deceleration phase, the specific feedback level (FBL) was between -20 and -200 (h g l-1)-1. The feedback components resulting from changes in glucose concentrations were calculated using experimentally determined values of mu max (0.55 +/- 0.05 h-1) and Ks (2.5 +/- 0.7 mg l-1). Only 0.1-40% of FBL could be accounted for by changes in glucose concentration, indicating the presence of additional growth regulators. The method developed may become a new tool for determination of growth-regulating cell-density factors in microbial cultures.