NK/ALAK cells allogeneic to the tumor targets mediate cytotoxicity superior to that of syngeneic or H2-matched NK/ALAK cells. IL-2-activated ALAK cells were prepared from B6, BALB/c, B10, and B10.D2 mice, and IL-2-activated NK cells were prepared from B6 SCID or C.B-17 SCID mice, as described in “Materials and methods.” (A-B) ⁵¹Cr-labeled RENCA (H2^d) or EL4 (H2^b) cells were cultured with B6 or BALB/c ALAK cells, and ⁵¹Cr-labeled C1498 cells were cultured with B10 or B10.D2 ALAK cells in triplicate for 4 hours, after which the level of cytotoxicity was measured, as described in “Materials and methods.” (C) P815 (H2^d) cells were cocultured with C.B-17 SCID or B6 SCID NK cells for 24 hours, and the inhibition of tumor growth was determined by colony assay, as described in “Materials and methods.” Representative data and SEMs from 4 independent experiments are shown. ^*P < .01 versus no NK and C.B-17 SCID NK groups.