Influenza nucleoprotein (NP) serves as a target antigen on abortively infected cells for cytotoxic T cells (Tc) cross-reactive for all type A influenza viruses, and it can also prime mice for such Tc. It is important to test the protective ability of NP-specific Tc clones in vivo in a productive influenza infection. In this report, we show that Tc clones of this antigenic specificity protect mice against a lethal influenza infection on transfer to syngeneic recipients, and also that they reduce virus titres in the lungs and trachea of mice challenged with homologous or heterologous type A influenza viruses. Simultaneous injection of IL-2 to maintain the viability of the Tc clones is not essential, but has made the clonal transfer experiments highly reproducible.