The effect of remantadine on penetration of influenza virus (the WSN strain) structures into MDCK cells was studied. Autoradiography of remantadine-treated cells infected with 3H-uridine-labeled virus showed remantadine to block the penetration of the parental viral structures from the perinuclear cytoplasm into the nuclei resulting in the structures aggregating in the perinuclear cytoplasm. Biophysical analysis of the viral structures recovered from the cells and analysis of their proteins by polyacrylamide gel electrophoresis showed that in remantadine-treated cells the amount of virus nucleoids (ribonucleoproteins, RNP, surrounded by a layer of M protein) increased in the perinuclear bytoplasm while the amount of intranuclear viral plasm while the amount of intranuclear viral RNPs decreased. These data suggest that the effect of remantadine is due to blocking of the second stage of deproteinization, that is, deproteinization of viral nucleoids to viral RNPs occuring in the perinuclear cytoplasm (possibly, on nuclear membrans); remantadine did not affect penetration of viral structures into the cells or the first stage of deproteinization (formation of nucleoids from virus particles). In order to inhibit deproteinization of viral structures remantadine should be present in the process of infection and be added soon after innoculation. When added 30 min after inoculation, remantadine has no effect on the intracellular distribution of viral structures.