Humic acid, a high-molecular-weight polyphenolic compound, exists abundantly in soil, natural water, and various terrestrial and aquatic environments. Humic acid causes peroxisome proliferation in mouse liver and induces the expression of peroxisome proliferator activated receptor (PPAR) in BNL CL.2 cells. Both cytotoxicity and flow cytometry show that humic acid inhibits the growth of C3H10T1/2 cells at G1 phase. C3H10T1/2 fibroblast cells express PPARgamma and the adipocyte P2 (aP2) genes which convert into adipocytes after being treated with humic acid. Our findings may provide a unique model for studying the molecular control of determination and differentiation of mesodermal cell lineages.