Abstract
After exposure of permissive cells to simian virus 40 (SV40), single particles were engulfed by the cell membrane and transported to the nucleus. The cell membrane closed tightly around the particles, increasing their diameter from 40 to 55 nm. The cell membrane was lost during interaction with the nuclear membranes, and particles of the original size were found in the nucleus 1 hr after infection. Uncoating of these nuclear particles occurred rapidly, and none could be found 4 hr after infection. Viral progeny appeared 24 hr after infection.