Effects of kochujang (Korean red pepper soybean paste) extracts on tumor formation, natural killer (NK) cell activity in spleen and glutathione S-transferase (GST) activity in liver were investigated in the sarcoma-180 cell transplanted mice. Inhibitory effects of these samples on lung metastasis of colon 26-M3.1 cells were also evaluated in the Balb/c mice. The injection of methanol extracts from traditional kochujang I (TK I, 0-day fermented), II (TKII, 6-month fermented), commercial kochujang (CK, 1-month fermented) and red pepper powder (RPP) significantly reduced tumor formation in Balb/c mice (p<0.05), TKII decreased tumor growth by 46% compared with control, resulting in the smallest tumor weight. The transplantation of sarcoma-180 cells increased the spleen/body weight ratio of Balb/c mice, while TKI and TKll significantly decreased this index (p<0.05). The effect of TKll and CK, fermented kochujang, on the NK cell activity of splenocytes was higher than that of sarcoma-180 cells transplanted control group. TK II recovered the activity of hepatic GST that was decreased by the transplantation of sarcoma- 180 cells in to the mice. All kochujang-treated mice had significantly fewer lung metastatic colonies than control mice. TKII was the most effective in inhibiting lung metastasis of colon 26-M3.1 cells. These results indicated that optimally ripened (6-month) TK had more suppressive effects on tumor formation and lung metastasis than RPP and kochujang without fermentation and commercially prepared kochujang in mice.