MDCK cells (a continuous line of dog kidney) were infected with 3H-uridine-labeled influenza virus, the WSN strain, and localization of parental structure in the cells was determined at various intervals after inoculation using the cell autoradiography method. At 15 and 30 min postinfection radioactive granules were found in nuclei and some of them concentrated around nucleoli. One hour after infection the granules were found both in nuclei and in cytoplasm. Two hours after infection the bulk of the granules was found in the cytoplasm localized in a limited poorly strained area of the cytoplasm contiguous to the nucleus. The parental structures recovered from the cytoplasm and the nucleus were identified by centrifugation in sucrose density and cesium chloride density gradients as viral ribonucleoproteins. The amount of the granules in the nuclei at early stages of infection increased proportionately to hemagglutinin precursor cleavage in the original virus. The cleavage was achieved by treating the virus or virus-producing cells with trypsin or allantoic fluid from noninfected chick embryos. The experimental results indicate that parental ribonucleo-proteins immediately after infection penetrate into the nucleus and then are transported into the cytoplasm.