NK cells, present in sponge allografts between all genetic barriers tested, are present at the graft site several days before alloimmune CTL are detected. Both NK and alloimmune CTL possess the AGM1 surface marker. Systemic as well as local treatment with AGM1 antisera depletes NK activity as well as alloimmune CTL at the graft site. NK cells may function at the graft site to facilitate development of alloimmune CTL and/or NK cells may themselves differentiate into alloimmune CTL.