Virus-immune cytotoxic T cells can inhibit effectively growth of vaccinia virus in acutely infected target cells in vitro by destroying infected target cells before infectious virus progeny is assembled. Together with the fact that virus-specific T cells are demonstrable after 3 days, very early during infection, and with strong circumstantial evidence from adoptive transfer models in vivo, these data suggest that in some virus infections T cells may in fact act cytolytically in vivo to prevent virus growth and spread and be an important early antiviral effector mechanism.