Partially purified homoserine dehydrogenase was prepared from pea seedlings. The optimum pH for this enzyme is approximately 5.4. The Kmvalues for ASA and TPNH are 4.6×l0−4Af and 7.7×l0−5M, respectively. This enzyme can also utilize DPNH but less effectively than TPNH. In contrast with yeast homoserine dehydrogenase which is insensitive to — SH reagents, the pea enzyme is inhibited almost completely by 10−4MPCMB and 10−5MHgCl2, the inhibition being removed by 10−2M thioglycolate. Homoserine dehydrogenase was found not only in decotylized seedlings, but also in cotyledons. The significance of this enzyme in homoserine biosynthesis in germinating pea seeds has been discussed.