Virus replication is important, but not necessary for NK cell activation and attenuation of B16lacZ lung metastases. PBS or indicated virus was injected i.v. into B6 mice. Twenty-four hours post–virus injection, spleens were harvested to assess (a) NK cell CD69 expression, (b) interferon-γ, (c) Granzyme B secretion via flow cytometry, and (d) ex vivo NK cytotoxicity (^P < 0.01; ^^P = 0.05 comparing treatment to PBS controls; the data are displayed as the mean percent (±SD) of chromium release from triplicate wells for the indicated E:T ratios). (e) B6 mice treated with PBS or indicated virus at day −1. At day 0, all mice received B16lacZ tumor cell inoculation. Mice were sacrificed at 3 days post–cell injection and the number of lung tumor metastases was quantified. Data are representative of three similar experiments with n = 4–6/group (*P < 0.05; **P < 0.01; ***P < 0.001; ns, not significant).