The prominent side effect of cyclosporin A, immunosuppressive agent, in oral tissues is gingival overgrowth(GO). It is characterized by the gingival enlargement with epithelial thickening, a large number of proliferating fibroblasts and overproduction of ECM components. Fibroblast accumulation in Cs A induced GO is caused by the Inhibition of apoptosis. But CsA effect on normal human oral keratinocyte(NHOK) remains unclear. Transglutaminase 2(TGase 2) which is expressed and activated in tissue where epithelial cells undergo apoptosis has been used as a marker for apoptotic cells. The purpose of this study were to study the effect of CsA on the proliferation and apoptosis of cultured NHOK by TGase 2 expression. After primary cultured NHOK was treated by 0.1, 1.0 and 10ug/ml Cs A, growth curve, MTT assay for succinyl dehydrogenase activity and RT-PCR for TGase 2 mRNA expression were done. The obtained results were as follows. MTT assay showed about 65% cell viability at 1.0μg/ml and 40% at 10μg/ml CsA. Growth curve showed normal S curve on control & DMSO, while decreased growth rate after 3 days of higher CsA tx. TGase 2 mRNA expression of cultured NHOK was the highest at 10μg/ml Cs A. TEM showed chromosome margination, and vacuole formation and clustered mitochodria in cytoplasm of cultured NHOK after CsA tx. It suggested that higher CsA might induce apoptosis of NHOK correlated with increased TGase 2 mRNA expression