By using DNA electrophoresis and propidium iodide (PI) staining flow cytometry (FACS) analysis, we studied the mechanisms of lymphokine-activated killer (LAK) cell-mediated cytotoxicity. In the presence of pokeweed mitogen (PWM), human LAK cells induced DNA fragmentation of two leukemic cell lines (U937 cells and Raji cells) and two solid tumor cell lines (SWIll6 cells and Hep-2 ceils), a hallmark of apoptosis. The reactions were carried out at the effector/target ratio of l:l in 4 h co-culture. Pretreatment with RNA and protein synthesis inhibitors (actinomycin D and cyclohexamide) did not prevent the target cells from apoptosis. As the TNF-resistant tumor cell lines such as SWlll6 cells and Raji cells were also triggered to apoptosis, other factors than TNF may play the role. DNA-PI staining FACS analysis also suggested that a part of LAK cells underwent apoptosis to some extent during incubation with target cells. The results provide a new way to investigate the mechanisms of the cytotoxicity of LAK cells, and a new possibility to enhance the efficiency of adoptive tumor therapy with LAK ceils.