BACKGROUND An excess storage of body fat causes obesity. Since obesity increases risk of chronic diseases, it is important to inhibit excessive storage of fat. Zaluzanin C is a sesquiterpene lactone isolated from Ainsliaea acerifolia. The aim of this study was to demonstrate the effect of zaluzanin C on differentiation of 3T3-L1 preadipocytes into mature adipocytes. METHODS The cytotoxicity of zaluzanin C and its effect on cell proliferation was determined. For the induction of adipocyte differentiation, 3T3-L1 preadipocytes were treated with differentiating medium containing 10 μg/mL insulin, 115 μg/mL methylisobutylxanthine, and 1 μM dexamethasone. Differentiated 3T3-L1 cells were subjected to Oil red O solution or used for Western blot analysis. Zaluzanin C was added to the cell culture medium at concentrations of 0, 1, 2.5, 5, and 10 μM. RESULTS Zaluzanin C did not inhibit cell proliferation and showed no cytotoxicity at 10 μM concentration in 3T3-L1 cells. Therefore, concentration range of 0-10 μM zaluzanin C was used for subsequent experiments. Zaluzanin C inhibited accumulation of lipid droplets in 3T3-L1 adipocytes. To understand the underlying mechanism of zaluzanin C, expression of adipogenesis regulators was determined by Western blot analysis. Zaluzanin C suppressed peroxisome proliferator-activated receptor gamma (PPARγ) expression, an adipogenesis related transcription factor, and inhibited aP2/fatty acid-binding protein-4 expression, a target gene of PPARγ. However, it did not affect expression of CCAAT/enhancer-binding protein alpha related with acquisition of insulin sensitivity. CONCLUSIONS These data suggest that inhibitory effect of zaluzanin C on adipogenesis of 3T3-L1 adipocytes could be partially caused by suppressing PPARγ.