In an attempt to elucidate the transfer and integration mechanism of Agrobacterium DNA upon crown gall induction, we translocated a borderless T-DNA to different sites of the C58 Ti plasmid. As a result of the physical linkage of the T-DNA onc genes with other Ti plasmid functions, the concerned strain retained tumor-inducing capacity. However, when the borderless T-DNA is separated on an independent replicon while all other pTi functions are provided in trans, the strain can no longer induce tumors on plants. We provide evidence that the right T-DNA border region harbors one or more in cis active functions essential in the transfer and/or stabilization of the T-DNA into plant cells. The strains used in these experiments allowed us to conclude that some function(s) of the Ti plasmid can induce plant cell proliferations independently of the T-DNA transformation event. The results described here indicate that other Ti plasmid sequences than solely the T-region can be transferred to plant cells.