The growth of Streptococcus salivarius subsp. thermophilus, Lactococcus lactis subsp. lactis, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus acidophilus and Lactobacillus paracasei was studied in a pH-auxostat. The computer-controlled smooth change of pH or temperature was used to study the effect of pH and temperature on the culture characteristics (specific growth rate mu growth yield Y(ATP) and specific lactate production rate Q(ATP)). The behaviour of mu and Q(ATP) with an increase of temperature suggested that ATP production capacity was an important factor in determining the maximum growth rate of lactic acid bacteria (LAB). The decrease of Y(ATP) with increase of temperature resulted in the decrease of specific growth rate while Q(ATP) remained constant or even increased. With the decrease of pH, decrease of both Y(ATP) and Q(ATP) was observed. S. thermophilus St20 having the highest maximum specific growth rate of 2.2 h(-1) at 44 degrees C was the most acid sensitive strain. The L. paracasei E1H3 having the lowest mu(max) was the most tolerant strain to pH change. The behaviour of mixed culture of L. bulgaricus and S. thermophilus in milk in a pH-auxostat with pH-decrease was in agreement with the pure culture experiments of the same species. The study showed that pH-auxostat with smooth controlled change of pH and temperature is an effective method for determination of technological characteristics and comparative physiological study of LAB.