Poliovirus-infected FL cells, incubated at 28°C, were examined by electron microscopy. The immunofluorescent antibody technique was employed for detection of the intracellular viral antigen. Electron micrographs of cells from infected cultures showed both extensive cytoplasmic and nuclear alterations. Virus crystals were observed not only in the cytoplasm but also in the nucleus of cells 14-20 hours after infection. Intracytoplasmic crystals consisted of dense particles, approximately 27 mμ in diameter, suggesting mature virus particles, while intranuclear crystals were composed of empty particles with a less dense central area. Nuclei were stained specifically by fluorescent antibodies as small brilliant foci, suggesting that the intranuclear crystals possess the same antigenicity as the viral antigen. As judged by their size, structure and antigenicity, the intranuclear virus-like particles probably represent an incomplete form of the virus. © 1969.