A kinetic model of the fermentative production of lactic acid from glucose by Lactococcus lactis ssp. lactis ATCC 19435 in whole-wheat flour has been developed. The model consists of terms for substrate and product inhibition as well as for the influence of pH and temperature. Experimental data from fermentation experiments under different physical conditions were used to fit and verify the model. Temperatures above 30 degrees C and pH levels below 6 enhanced the formation of byproducts and D-lactic acid. By-products were formed in the presence of maltose only, whereas D-lactic acid was formed independently of the presence of maltose although the amount formed was greater when maltose was present. The lactic acid productivity was highest between 33 degrees C and 35 degrees C and at pH 6. In the concentration interval studied (up to 180 g l(-1) glucose and 89 g l(-1) lactic acid) simulations showed that both substances were inhibiting. Glucose inhibition was small compared with the inhibition due to lactic acid.