Antitumor activities of a peptide fraction isolated from traditional Korean soy sauce (SSP) were investigated in vitro and in vivo using cancer cell lines and F9 teratocarcinoma-bearing BALB/c mice. SSP exerted a dose-dependent antiproliferative effect on P388D1 mouse lymphoma, F9 mouse teratocarcinoma, and DLD-l human colon cancer cells with $IC_{50}$ values of 11, 50, and $50mug/ml$, respectively. Tumor growth in F9 teratocarcinoma-bearing BALB/c mice, orally administered with 80 and 200 mg/kg/day of SSPs, was inhibited 10.3% and 52.4%, respectively, and survival days increased by 11.9% and 22.1%, respectively, compared to the control group. The results of antitumor activities exerted by SSP in vitro and in vivo suggest the feasibility of using SSP as an antitumor agent.