The ubiquitous grapevine-associated octopine/cucumopine Ti plasmids of biotype III Agrobacterium tumefaciens strains carry two T regions, TA and TB, with a complex oncogene arrangement. Within the octopine/cucumopine group, two main strain types were identified: 'large TA' strains with a TA region resembling the TL region of the biotype I octopine strain Ach5 and 'small TA' strains with a similar T region organization as the 'large TA' strains but with a large internal TA deletion. Structural and functional studies of the representative 'large TA' strain Tm4 revealed six oncogenes. Each oncogene was inserted in a disarmed vector and tested for biological activity using the corresponding oncogenes of Ach5 as standards. Five Tm4 oncogenes, TA-iaaM, T-ipt, T-6b, TB-iaaH and TB-iaaM, were shown to be active, the IS-interrupted TA-iaaH gene was inactive. To study the role of each gene in the pTiTm4 context, several single and multiple pTiTm4 mutations were constructed. It was shown that whereas TA-iaaM and TB-iaaH are essential for tumour formation on grapevine, T-ipt, T-6b and TB-iaaM are not. The avirulence of the TA-iaaM- mutant was shown to be due to an inhibitory effect of the T-ipt gene, since a TA-iaaM-/T-ipt- double mutant was fully virulent. We conclude that the TA-iaaM gene of 'large TA' strains is specifically required to counteract the tumour growth inhibiting activity of the T-ipt gene. Both TA-iaaM and T-ipt are absent from the 'small TA' strains. A model on the roles and interactions of the different oncogenes in 'large TA' and 'small TA' strains is presented.