Acetolactate synthase (ALS, EC 126.96.36.199) has been extracted and partially purified from etiolated barley shoots (Hordeum vulgare L.). Multiple forms of this enzyme were separated by gel filtration and/or anion-exchange chromatography using fast protein liquid chromatography. It could be demonstrated that these two species are in equilibrium, which strongly depends on the structural role of flavin adenine dinucleotide and pyruvate. With 50 micromolar of flavin adenine dinucleotide in the medium most of the ALS aggregates as a high molecular weight form (Mr = 440,000), while 50 millimolar pyruvate facilitates dissociation into the smaller form (Mr = 200,000). Data are presented to show that two enzymatically active forms are not isozymes but different oligomeric species or aggregates of the basic 58-kilodalton subunit of ALS. These different ALS species exhibit little difference in feedback inhibition by valine, leucine and isoleucine or in inhibition by the sulfonylurea herbicide chlorsulfuron. Both aggregation forms show a broad pH-optimum between 6.5 and 7. Furthermore, the affinity for pyruvate and the amount of directly-formed acetoin indicate similar properties of these separated ALS forms.