We report here that the normal cellular protein CC3/TIP30, when in excess, inhibits nuclear import in vitro and in vivo. CC3 binds directly to the karyopherins of the importin β family in a RanGTP-insensitive manner and associates with nucleoporins in vivo. CC3 inhibits the nuclear import of proteins possessing either the classical nuclear localization signal or the M9 signal recognized by transportin. CC3 also inhibits nuclear translocation of transportin itself. Cells modified to express higher levels of CC3 have a slower rate of nuclear import and, as described earlier, show an increased sensitivity to death signals. A mutant CC3 protein lacking proapoptotic activity has a lower affinity for transportin, is displaced from it by RanGTP, and fails to inhibit nuclear import in vitro and in vivo. Together, our results support a correlation between the ability of CC3 to form a RanGTP-resistant complex with importins, inhibit nuclear import, and induce apoptosis. Significantly, a dominant-negative form of importin β1 shown previously to inhibit multiple transport pathways induces rapid cell death, strongly indicating that inhibition of nuclear transport serves as a potent apoptotic signal.