1. 1. A method is described for the complete separation of 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase (7-phospho-2-keto-3-deoxy-d-arabino-heptonate d-erythrose-4-phosphate-lyase (pyruvate-phosphorylating), EC 220.127.116.11) isoenzymes inhibited by tryptophan (DAHP synthase (Trp)), phenylalanine (DAHP synthase (Phe)) and tyrosine (DAHP synthase (Tyr)). 2. 2. DAHP synthase (Tyr) is purified to homogeneity as judged by electrophoresis on polyacrylamide gels. The presence of the substrate phosphoenolpyruvate is necessary for stability during purification and subsequent storage. 3. 3. The purification procedure makes use of allosteric ligand dependent reversible changes of molecular weight. Purification by ligand-dependent changes of molecular weight may have wider applications.