Aqueous extract of acetone-dried baker''s yeast is acidified to pH 4.8 for pptn. off the impurities. The supernatant after centrifugation is fractionated with ammonium sulfate at 0-45. 45-65, and 65-100% saturations. The 1st fraction Enzyme I. is further purified with ammonium sulfate between 0-45% saturation. The 2nd one Enzyme II is fractionated at 45-50% and 50-65% saturations. With the use of dialyzed prepn. of I and II, it is shown that Enz. I catalyzes the reaction of homoserine with ATP to form an intermediate, phospho-homoserine, in the presence of Mg++, while II catalyzes the subsequent reaction leading to threonine production. The occurrence of the intermediate is confirmed by paper chromato- graphic analysis (Rf. 0.14) with a phenol-water mixture.