A bioactive factor inhibiting adipocyte differentiation was found in the water-soluble fraction of Maitake (Grifola frondosa). The fraction including bioactive factors was isolated by HPLC on Superdex Peptide HR 10/30. This water-soluble fraction was found to inhibit the differentiation of adipocytes as determined by the decrease in the glycerol-3-phosphate dehydrogenase (GPDH) activity, triglyceride (TG) accumulation and leptin levels. Furthermore, this factor inhibited the expression of peroxisome proliferator activated receptor γ(PPARγ), one of the master regulators expressed early in adipocyte differentiation cascades. Thus, this bioactive factor may act on the expression of some factors at earlier stages leading to the expression of PPARγ.