It has been previously reported that a short treatment of human cervix carcinoma HeLa cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) induced apoptosis. We examined the action of MNNG on HeLa cells and compared it with that of cisplatin. MNNG damaged the integrity of the cell membrane and killed the cells within 3 hours. During this period no changes characteristic for apoptosis, such as cell shrinkage, condensation of nuclei, chromatin fragmentation or activation of caspases, could be detected. However, the exposure of HeLa cells to 50 micro M MNNG for 1 h resulted in dramatic DNA damage. The MNNG-induced disruption of cell membrane associated with cell death indicates that HeLa cells die by necrosis.