In the present work, the influences of ethanol (fermentation substrate) and acetic acid (fermentation product) concentrations on the starting-up of a pilot plant acetifier are evaluated. Some effects of activation and inhibition acting on bacterial growth of acetic acid bacteria are registered as a function of the concentrations of substrate and product. As a consequence of the related studies, an optimum starting-up protocol has been proposed. In this, the volumes of wine and inoculum that must be added at the beginning of the process, together with the appropriate instant to make the additions of fresh wine, are established. This protocol is perfectly scalable to other industrial acetifiers, following the proposed stages and addition sequences. (C) 2002 Elsevier Science Ltd. All rights reserved.