Despite the widespread use of Agrobacterium tumefaciens to transfer genes into plant systems, host responses to this plant pathogen are not well understood. The present study shows that disarmed strains of Agrobacterium induce distinct host responses when infiltrated into leaves of Nicotiana tabacum. The responses are limited to the infiltrated zone and consist of i) induction of pathogenesis-related (PR) gene PR-1 expression and resistance to subsequent infection with tobacco mosaic virus, ii) chlorosis and loss of chloroplast rRNAs, and iii) inhibition of leaf expansion. Induction of the latter two sets of responses depends on the age of the leaf and is most apparent in young leaves. Strains with or without binary vectors induce all the responses, showing that DNA transfer is neither required nor inhibitory. A. tumefaciens cured of the tumor-inducing (Ti) plasmid is slightly defective for induction of the three responses, showing that Ti plasmid-encoded factors produced by the disarmed strains contribute only slightly. However, T-DNA-encoded factors alter at least one of the host responses, because infiltration with the oncogenic strain C58 induced more pronounced chlorosis than the disarmed control. Auxin is one of the T-DNA products responsible for disease induction by oncogenic A. tumefaciens. We found that C58-infiltrated zones-but not those infiltrated with the disarmed control-have increased levels of miR393, a microRNA that represses auxin signaling and contributes to antibacterial resistance.