Sodium fluoride (NaF), an inhibitor of membrane activity, was used to investigate the mechanisms by which alloimmune macrophages kill target cells and produce plaques in monolayers of specific target cells. When target cells were treated with 0.02 M or 0.002 M NaF and washed before adding alloimmune macrophages to establish loci of cell interaction, plaque formation was not affected. However, when interacting alloimmune macrophages and target cells were initially exposed for 30 minutes or 1 hour to 0.02 M or 0.002 M NaF plaque formation was markedly suppressed. Comparative studies with NaF and the inhibitor of protein synthesis, Puromycin, showed that protein synthesis by macrophages was not affected by treatment with 0.002 M NaF. Results obtained by subjecting interacting cells to 1 hour exposure to NaF at various intervals indicated that the destruction of target cells by immune macrophages was irreversibly initiated in large measure during the first hour of cell interaction. It is postulated that the membrane activity of immune macrophages responsible for target cell killing involves the release or secretion of a cytoxin.