Streptococcus cremoris AM2 is characterized by an aminopeptidase and Leuconostoc lactis CNRZ 1091 by an α-galactosidase and a citrate lyase. These strains were grown in pure or mixed cultures, in presence or absence of citrate (15 mM) and at controlled or uncontrolled pH. Cell populations and the activities of the enzymes were measured during microbial growth. Linear correlations were established between the population of S. cremoris AM2 and aminopeptidase activity, and between that of L. lactis CNRZ 1091 and the activities of α-galactosidase and citrate lyase. These correlations held regardless of whether the culture was pure or mixed and if the pH was controlled or not. The presence of citrate did not change citrate lyase and aminopeptidase activities, but inhibited the synthesis of the agr-galactosidase and not its activity. The linear relationships permit the determination of bacterial populations in less than 2 h without counting but by measuring enzyme activities.