Mutation of the genes virA, virB, virC, and virG of the Agrobacterium tumefaciens octopine-type Ti plasmid pTiR10 was found to cause a 100- to 10,000-fold decrease in the frequency of conjugal transfer of this plasmid between Agrobacterium cells. This effect was not absolute, however, in that it occurred only during early times (18 to 24 h) of induction of the conjugal transfer apparatus by octopine. Induction of these mutant Agrobacterium strains by octopine for longer periods (48 to 72 h) resulted in a normal conjugal transfer frequency. The effect of these vir gene mutations upon conjugation could be restored by the introduction of cosmids harboring wild-type copies of the corresponding disrupted vir genes into the mutant Agrobacterium strains. In addition, transfer of the self-mobilizable plasmid pPH1JI was not impaired in any of the mutant Agrobacterium strains tested. The effect of vir gene function on the conjugal transfer of the Ti plasmid suggests that a relationship may exist between the processes that control the transfer of the T-DNA from Agrobacterium to plant cells and the conjugal transfer of the Ti plasmid between bacterial cells.