BACKGROUND: Interferon-gamma has various biologic effects, including antiviral effect, antitumor proliferative effect, activation of macrophage and B lymphocyte, and increased expression of major histocompatibility complex. Especially, antitumor proliferative effect of interferon-gamma has already been proved to be important in vivo as well as in vitro. And, clinical studies of interferon-gamma have been tried in lung cancer patients. However, the mechanism of antitumor effect of interferon-gamma has not yet been established despite of many hypotheses. "Necrosis" is a type of cell death which is well known to occur in the circumstances of severe stresses. In contrast, "apoptosis" is another type of cell death which occurs in such biological circumstances as embryonic development, regression of organs, and self-tolerance of lymphocytes. And, apoptosis is an active process of cell death in which cells are dying with fragmentations of their cytoplasms and nuclei. And, in the process of apoptosis the DNAs of cells are cleaved between nucleosomes by unidentified endonuclease and therefore DNAs of apoptotic cells result in a typical electorphoresis pattern known as DNA ladder pattern. Recently it has been suggested that cytotoxic effect of interferon-gamma occurs via apoptosis. To elucidate the mechanism of antitumor cytotoxic effect of interferon-gamma, we microscopically observed a lung cancer cell line, A549 which was treated with interferon-gamma. We observed A545 treated with interferon-gamma was dying fragmented. And so, we performed this study to find out that the mechanism of antitumor cytotoxic effect of interferon-gamma be apoptosis. METHOD: We treated A549, human lung cancer cell line with various concentration of interferon-gamma and quantified its cytotoxic effect of various periods, 24 hours, 72 hours and,, 120 hours by MTT(dimethylthiazoly1 diphenyltetrazolium bromide) bioassay. Also, after we treated A549 with 100 units/ml of interferon-gamma for 120 hours, we observed the pattern of cell death with inverted microscope and we extracted DNAs from the dead A549 cells and observed the pattern of 1.5% agarose gel electrophoresis with ethidium bromide staining. RESULT: 1) Cytotoxic effect of interferon-gamma on A549: For the first 24 hours, threre was little cytotoxic effect and for between 24 hours and 72 hours, there was the beginning of cytotoxic effect and for 120 hours there was increased cytotoxic effect. 2) Pattern of A549 cell death by interferon-gamma: We observed with inverted microscope that A549 cells were dying fragmented. 3) DNA ladder pattern of gel electrophoresis: We observed DNA ladder pattern of gel electrophoresis of extracted DNAs from dead A549 cells. CONCLUSION: We concluded that the mechanism of interferon-gamma induced cytotoxicity on lung cancer cell line, A549 be via apoptosis.