Upon penetration of the influenza virus nucleocapsid into the host cell cytoplasm, the viral RNA and associated proteins are transported to the nucleus, where viral transcription and replication occur. By using quantitative confocal microscopy, we have found that over half of cell-associated nucleoprotein (NP) entered the nucleus with a half time of 10 min after penetration into CHO cells. Microinjection and immunoelectron microscopy experiments indicated that the NP entered the nucleus through the nuclear pore as part of an intact ribonucleoprotein (RNP) structure and that its transport was an active process. Transport of the incoming RNPs into the nucleus was not dependent on an intact microfilament, microtubule, or intermediate filament network. Subsequent to penetration, the matrix (M1) protein appeared to dissociate from the RNP structure and to enter the nucleus independently of the RNP. We found that 50% of penetrated M1 entered the nucleus with a half time of 25 min after penetration into CHO cells. Nuclear transport of M1 appeared to occur by passive diffusion. Entry of incoming M1 into the nucleus was not a prerequisite for infection.