Doenjang (fermented soy paste) is a Korean traditional fermented food. In previous studies, we demonstrated that Doenjang extracts exhibited anti-mutagenic and anti-carcinogenic activities. Here we investigated the effects of Doenjang hexane fraction (DHF) on cell cycle progression in the human breast carcinoma MCF-7 cells. Treatment of DHF to MCF-7 cells induced a G1 phase arrest of the cell cycle, which correlated with the accumulation of the hypophosphorylated form of the retinoblastoma protein (pRB) and enhanced association of pRB with the transcription factor E2F-1. After DHF treatment, the expression of D-type cyclins was decreased in a time-dependent manner, but DHF did not affect the levels of cyclin-dependent kinases (Cdks), cyclin E and cyclin A protein. However, the activity of Cdk2 and cyclin E-associated kinase was decreased in a time-dependent manner. The tumor suppressor p53 and Cdk inhibitor p21, a known downstream effector of the p53, and association of p21 with Cdk2 were markedly induced in DHF-treated cells. Taken together, the present results indicate that treatment of MCF-7 cells with DHF induces a significant inhibition of pRB phosphorylation and increase of Cdk inhibitor p21 that appear to be responsible for the observed G1 arrest.