MDCK cells (a continuous cell line of dog kidney) were infected with 3H-uridine- or 14C-aminoacide-labeled influenza virus, the WSN strain. The cells were fractionated 30 min and 2 hours postinfection, and viral structures in subcellular structures were identified by sedimentation and density centrifugation and protein analysis in polyacrylamide gel. At 30 min after inoculation the perinuclear cytoplasm was shown to contain structures with the properties of viral nucleoids (ribonucleoproteins--RNP--surrounded with a layer of M protein). Their sedementation rate in glycerol gradients was 70--90 S, and the buoyant density in cesium chloride 1.30 g/ml. The nuclei were found to contain viral RNP with a buoyant density of 1.39--1.41 g/ml. Two hours after inoculation the amount of RNP in the nuclei decreased and in the cytoplasm increased indicating RNP transport from the nucleus into the cytoplasm. Treatment of the cells with cytochalasine B (a substance breaking nucleus-cytoplasm bonds) resulted in redistribution of radioactivity in subcellular fractions: an increase of radioactivity in the nuclear extract and a decrease in the nuclear sediment. Autoradiography of cytochalasine-treated cells revealed accumulations of viral structures around nucleoli. These data suggest participation of the nucleoli in transportation of viral structures into the cytoplasm. Based on the foregoing data, a hypothetical scheme of the processes of deproteinization and transport of viral structures in an infected cell has been proposed.