The NPN3A mutation was engineered into PR8 (H1N1) and HKx31 (H3N2) influenza viruses (PR8NPN3A, HKNPN3A) to allow cross-challenge experiments in the absence of antibody neutralisation. The B6 mice were immunised i.p. with the virulent PR8 mutant and wt viruses, while the HK viruses were used for primary (1 o ) i.n. infection of naïve mice or secondary (2 o ) i.n. challenge of PR8-immune (.30d previously) mice. Naïve (primary) or PR8-immune (PR8, or PR8NPN3A) mice were challenged i.n. with the homologous virus (HK, or HKNPN3A) and CD8 + T cell responses were measured using the D b NP 366 and D b NPN3A tetramers (Fig 2) . It was immediately apparent that the splenic D b NPN3A + CD8 + set elicited by the HKNPN3A challenge was significantly smaller on d10 (p,0.05) than the D b NP 366 + CD8 + T cell response induced by the wt virus (Fig 2B) , though there was no significant difference between D b NPN3A + CD8 + and D b NP 366 + CD8 + T cell numbers at the site of infection (BAL, Fig 2A) . This has been seen before [24] and suggests that the need to clear virus from the lung results in preferential CTL localization to the site of infection when immune T cell numbers are limited. The profile of a diminished D b NPN3A + CD8 + T cell response was maintained into memory (d28, Fig 1C; p,0.02), supporting the view that the relative size of persistent T cell pools reflects the extent of antigen driven proliferation during the acute anti-viral response.