Infection of maize seedlings by Ustilago maydis results in neoplasia. The response of host nuclei and nucleoli to infection in these neoplasms has been examined in detail by Feulgen microspectrophotometry and quantitative interferometry. Seven days after infection, two distinct tissue zones could be recognized within the neoplastic leaf sheaths. Small cells around the vascular stele contained nuclei of 2C and 4C DNA content suggesting a meristematic function. Larger, vacuolate cells that form the bulk of the tissue contained nuclei of 16C, 32C, and 64C DNA levels, indicating substantial endopolyploidy. Nucleolar dry mass was substantially increased in infected cells (up to ten-fold compared with controls), indicating enhanced ribosome synthesis. These results are discussed in relation to the hormonal control of DNA synthesis and mitosis.