Cells enriched for NK activity (poly I:C induced, x-ray resistant, and nonadherent), include two phenotypically and functionally different populations. Both populations of NK cells are AGM1+, Ly-1.1-, Ly-2.1-, Ia-, and have the morphology of large granular lymphocytes. One population, however, is Thy-1+ while the second population is Thy-1-. Thy-1+ NK cells lyse YAC-1 and P815 target cells; Thy-1- NK cells lyse YAC-1 but not P815 target cells. The FACS was used to obtain homogeneous populations of Thy-1+ and Thy-1- NK cells, which retain high cytotoxicity. While Thy-1- NK cells suppress the antibody response in vitro by suppressing or eliminating DC, Thy-1+ NK cells do not suppress antibody responses in vitro.