Uncoating of influenza virus (strain WSN) in MDCK cells was studied by following the fate of the virus labelled with radioactive precursors. The accumulation of subviral components of input virus was observed in nuclear-associated cytoplasm (NAC) obtained by treatment of the nuclei with citric acid. Two types of subviral components were found there, ribonucleoproteins (RNPs) and larger subviral particles (SVP) containing RNPs in association with M protein. SVP, with different relative amounts of M protein, were revealed in NAC, suggesting that M protein was gradually released from RNPs. The released RNPs entered the nuclei while M protein accumulated within perinuclear membranes. Thus, SVP could be regarded as probable intermediates in virus uncoating. Rimantadine prevented the release of M protein from RNPs and their penetration into the nuclei provoking the accumulation of subviral components in NAC.