NK cells have impaired IFN-γ production but normal cytotoxicity in a tumor-bearing host. Splenocytes from control and EL4-bearing mice that were injected with LPS were tested for IFN-γ production and cytotoxicity. (A) NK1.1⁺CD3^- splenocytes were evaluated for CD11b expression and IFN-γ production. (B) The mean ± SEM of the NK-cell phenotype. The NK-cell populations CD11b⁺IFN-γ⁺ (P ≤ .002) and CD11b^-IFN-γ^- (P < .001) were significantly decreased and increased, respectively, in tumor-bearing mice compared with controls. (C) Splenocytes were tested for cytotoxic potential against the NK-sensitive target Yac-1. The E/T ratios were adjusted based on the percentage of NK cells in the spleen. The experiment is representative of 3 separate experiments. (D) EL4 tumor-bearing mice had diminished NK immunity toward tumor challenge. Control and EL4 tumor-bearing mice were challenged with 2.5 × 10⁶ RMA-S cells subcutaneously injected at day 13 after EL4 challenge (n = 5). The sizes (left) and incidence (right) of RMA-S tumor are measured by physical examination. Size data shown are the mean ± SEM of tumor diameters (P = .004).