Defensins induce ion channels in model lipid bilayers and permeabilize the membranes of Escherichia coli. We investigated whether similar membrane-active events occur during defensin-mediated cytolysis of tumor cells. Although defensin-treated K562 targets did not release chromium-labeled cytoplasmic components for 5-6 h, they experienced a rapid collapse (within minutes) of the membrane potential, efflux of rubidium, and influx of trypan blue. Defensin treatment also blunted the subsequent acidification response induced by nigericin, thereby further supporting the notion of enhanced transmembrane ion flow during exposure. These initial effects on the plasma membrane were not sufficient for subsequent lysis; a second phase of injury was required which involved the continued presence of defensin. The rapid membrane permeabilization phase was inhibited by azide/2-deoxyglucose, cytochalasin B, and increased concentrations of extracellular potassium and was unaffected by actinomycin-D, cycloheximide, and varying the calcium concentration. In contrast, the second phase was unaffected by cytochalasin B, inhibited by azide/2-deoxyglucose, enhanced by actinomycin D and cycloheximide, and varied with calcium concentration. These results indicate the initial adverse effect of defensins on mammalian cells occurs at the cell membrane. It is possible that the second phase of injury is mediated intracellularly by defensin that has been internalized through this leaky membrane.