This invention relates to several plasmids which are useful for genetically transforming plant cells. A first plasmid, such as pMON120, contains a T-DNA border, one or more marker genes, a unique cleavage site, and a region of Ti plasmid homology. A gene which is expressed in plant cells may be inserted into this plasmid to obtain a derivative plasmid, such as pMON128 which expresses neomycin phosphotransferase in plant cells. The derivative plasmid is inserted into a suitable microorganism, such as A. tumefaciens which contains a Ti plasmid. The inserted plasmids recombine with Ti plasmids to form co-integrate plasmids. Only a single crossover event is required to create the desired co-integrate plasmid. A. tumefaciens cells with co-integrate plasmids are selected and co-cultured with plant cells. The co-integrate Ti plasmids enter the plant cells and insert a segment of T-DNA which does not contain tumorigenic genes into the plant genome. The transformed plant cell(s) may be regenerated into a morphologically normal plant which will pass the inserted gene(s) to its descendants.