0730 09 1. l-Threonine dehydratase (E.C. 4.2.1.16) activity in the cell-free extracts of the trophozoites of Hartmannella culbertsoni has been purified 20-fold. 2. The enzyme requires pyridoxal-5-phosphate for its activity and is not cold-labile. Kinetic studies have shown that there is homotropic cooperative interaction between the substrate molecules. The enzyme is susceptible to end-product regulation, l-isoleucine being a feed back inhibitor and l-valine can partially reactivate the l-isoleucine inhibited enzyme. Some nucleotides stimulate the activity of the enzyme indicating allosteric nature of the enzyme. 3. Certain amoebicidal drugs and antibiotics have marked inhibitory effect on the enzyme.