A new murine natural killer (NK) cell alloantigen, NK-3.1, has been identified by analysis of the complement dependent anti-NK cell activity of C3H anti-ST and BALB/c anti-DBA/2 alloantisera. Flow cytometric studies with a specific antiserum made in backcross mice indicate that this alloantigen is expressed by 2-3% of the spleen cells of NZB and C57BL/6 mice, positive strains, but is undetectable on the spleen cells of C3H mice, a negative strain. Since NK-1.1 and NK-2.1 are also expressed on 2-3% of spleen cells of positive strains we suggest that all three alloantigens identify the NK cells in normal mouse spleen which lyse the lymphoma target cell line YAC.