In the cytoplasms of chick embryo fibroblast and Ehrlich ascitic carcinoma cells infected with influenza virus (fowl plague virus), in addition to fragmented virus nucleocapsid larger nucleocapsid structures were found which sedimented in the region of 90 -120S. The structures were detected upon short 3H-uridine label of the cells. Their buoyant density in cesium chloride was higher than that of the fragmented nucleocapsid (1.34 -1.39 g/cm3). In electron microscope, the structures were visualized as thin nonhelical filaments 3.5 nm in diameter, their morphology being no different from that of a similar rapidly sedimenting structure isolated from the nucleoplasms of the same cells. To determine the possibility of transfer of the rapidly sedimenting structure from the nucleus into the cytoplasm, a cell-free system was used containing nuclei from influenza virus-infected cells labeled with 3H-uridine for 5 min, as well as the cytoplasm from uninfected and unlabeled cells. The presence of a labeled rapidly sedimenting structure in the cytoplasm of the cell-free system suggests that the structure is synthesized in the nucleus and then transported into the cytoplasm. The relation of this structure to the fragmented nucleocapsid is unknown. It may be assumed to be its intracellular precursor.