DNA recombination using plasmid pGA658, a Ti plasmid vector, was tried to study the role of yeast homoserine dehydrogenase after introducing its gene into plant cells. Two recombinant plasmids with opposite orientation of the gene DNA under nos promoter of vector pGA658, pKDB1 and pKDB2, were constructed by the aid of two intermediate vectors, pUC7 and pUC119. Transformed E. coli and Agrobacterium tumefaciens were confirmed by the resistance to appropriate antibiotics and by sizing DNA restriction fragments after plasmid isolation. Assay of homoserine dehydrogenase activity showed that a little increase in its activity was detected in transformed E. coli. but no increase in transformed Agrobacterium tumefaciens, compared with its untransformed strain.