Tumorigenic and non-tumorigenic strains of Agrobacterium tumefaciens were labelled with tritiated thymidine and added separately to suspension cultures of X-D line habituated tobacco cells. The subsequent association of bacteria with plant host cells and the passage of bacteria and bacterial DNA into these cells was examined by conventional light and electron microscopy and light microscope autoradiography. The results suggest that both strains of bacterium enter damaged host cells (via wall lesions), where they remain restricted to cell vacuoles. By 48 h, the radioactive label has spread from the vacuoles of infiltrated cells to the non-vacuolar cytoplasm of the whole callus tissue. Tumorigenic bacteria differ from the non-tumorigenic strain in promoting a significant increase in host cell nuclear volume, which is paralleled by clear and specific nuclear labelling. It is proposed that tumorigenic DNA, but not non-tumorigenic DNA, is specifically incorporated into, and activates, the host cell nucleus.