Enzymes catalyzing the synthesis and subsequent transformation of α-acetolactate (AcL)—acetolactate synthase (AcLS) and acetolactate decarboxylase (AcLDC)—were isolated and partially purified from the cells of lactic acid bacteriaLactococcus lactis ssp.lactis biovar.diacetylactis, strain 4. The preparation of AcLS, purified 560-fold, had a specific activity of 358 300 U/mg protein (9% yield). The preparation of AcLDC., purified 4828-fold, had a specific activity of 140 U/mg protein (4.8% yield). The enzymes exhibited optimum activity at pH 6.5 and 6.0, respectively (medium, phosphate buffer). The values of apparentK m, determined for AcLS and AcLDC with pyruvate and AcL, respectively, were equal to 70 mM and 20 mM. AcLS appeared as an allosteric enzyme with low affinity for the substrate and a sigmoid dependence of the activity on the substrate concentration. In the case of AcLDC, this dependence was hyperbolic and the affinity of the enzyme for its substrate was high (K m = 20 mM). Leucine, valine, and isoleucine were shown to be activators of AcDLC.