Abstract The cytoplasmic surface (PS) of the plasma membrane of cells infected with Sendai virus was studied by immunofluorescence microscopy and freeze-drying electron microscopy. After cells had been attached to glass coverslips, they were subjected to a jet stream of physiological buffer which sheared off the upper portion of each cell, leaving the attached membrane with the PS exposed. This uncapping maneuver permitted direct examination of internal virus-specific elements associated with the inner surface of the host cell. At a stage of infection at which viral budding occurs, strands of nucleoprotein (RNP) were observed to be attached to the PS of plasma membranes. The sites at which RNP was adherent to the membrane were modified by virus-specific particles arranged in orthogonal patterns. The presence of the same crystalline structures in the hydrophobic domain of freeze-fractured membranes indicated that they were inserted into the inner lipid leaflet. The spatial association of the surface glycoprotein spikes and the internal RNP with this crystalline structure suggests its special relationship if not identity with the internal viral matrix (M) protein. The possible significance of the localization and crystalline nature of this structural element with respect to viral morphogenesis, hemolytic and cell-fusing activities is discussed. In contrast to the foregoing changes observed in the infected cell, no detectable viral antigens were found on the PS of normal cells to which exogenous virions had been fused. Absence of internalized antigen from the PS under these circumstances could indicate that infectious viral components are processed by the potential host cell in a manner which differs from what is observed with human erythrocytes. In the latter instance internalized antigens after fusion of virus to the cell remain associated with the PS.