Interferon preparations, in addition to their antiviral properties, may inhibit cell growth and multiplication, enhance the expression of cell surface antigens and influence some functions of T-1ymphocytes, macrophages and natural killer cells. The study was performed in order to investigate the cytolytic effect, antiproli ferative effect and HLA antigen expression effect of the gamma-interferon(IFN- r ) on human melanoma cell line A-375. the cytolytic activities were checked by 'Cr release assay, the antiproliferative activities were analyzed by the 'H-thymidine uptake test and HLA antigen expressions were observed by the indirect imrnuno fluor esent method. The results were as follows : 1. The cytolytic effect of the peripheral rnononuclea.r cells treated with INF- on A 375 human melanoma cell line was increased to 31.4%, 36.5%, 33.9%, 53.9 %, 13.9% respectively in the experiment 1, 2, 3, 4, 5 as compared to control group (-11.4%, 3.6%, 18.5%, 35.5%, 6.7% respectively). 2. 1ri the dose response check of the peripheral mononuclear cells treated with INF r to A 375, the cytolytic effect was definitely observed at 200U/ml concentration of the INF r and maximal effect was observed at 500U/ml concentration. 3. The IIUF r treated natural killer cells did not show any significant increase in cytolytie activity as compared to that of untreated natural killer cell and the same results were obtained with monocytes. This cytolytic activity was significantly increased when INF-r treated natural killer cells were co-culturd with monoeytes. 4. The cyaolytjc activity of natural killer cells was increased at the same degree when using culture supernant of INF- treated monocytes as in the case of using