The development of a new turbidimetric method to estimate bacterial growth in heterogeneous media using Bacillus subtilis (ATCC 6633) as a model is described. A useful equation for actual micro-organism absorbance in a heterogeneous culture medium of Torula yeast at 1% with a minimum error of 1.22% was obtained with the application of the absorbance additivity principle. Consequently, B. subtilis ATCC 6633 growth kinetics could be studied in a Torula yeast suspension, showing a similar kinetic behaviour to that observed during cultivation in nutrient broth. (C) 2002 Elsevier Science Ltd. All rights reserved.