Blocking inhibitory receptors on allogeneic ALAK cells augments the inhibition of tumor growth in vitro. B10 or B10.D2 ALAK cells were preincubated with F(ab′)₂ fragments of normal mouse IgG, anti-Ly49C/I (5E6), or anti-Ly49G2 (4D11) antibodies for 2 hours. C1498 and P815 tumor cells were cultured with pretreated ALAK cells for 20 to 24 hours, after which cocultured cells were replated in 96-well, flat-bottomed microtiter plates at various numbers of cells per well in triplicate, as described in “Materials and methods.” The level of cell proliferation was determined using [³H]-thymidine at 1 μCi/well (0.037 MBq), as described in “Materials and methods.” C1498 (A) and P815 (B) cells were plated at 3000 or 30 cells per well, respectively, based on the cell numbers at the initiation of coculture, and cell proliferation was assessed at day 3 in culture. [³H]-thymidine incorporation of C1498- and P815-only control was 1.2 × 10⁵ cpm and 7.2 × 10⁴ cpm, respectively. Representative data from 3 independent experiments are shown, and statistical differences between various antibody treatments were determined by the unpaired t test with Welch correction. ^*Significant difference compared with control antibodies. ^**Significant difference compared with H2-matched ALAK cells. Error bars indicate SEM.