Ethanol extracts of the whole herb of Agastachis Herba (A) and of Pueraria Radix (P) alone and of their mixture (A+P) downregulated the cell growth, cyclooxygenase-2 (COX-2) expression, prostaglandin $E_2;(PGE_2)$, and cGMP production. A, P, and A + P inhibited the cell growth of HT-29 colon cancer cells in a concentration- and time-dependent manner but not the growth of normal colon cell, CCD-112CoN. In addition, they markedly inhibited the productions of $PGE_2$ and cGMP as well as the mRNA expression of COX-2. These data suggest that non-toxic concentration of A, P, and A + P have a significant effect on the in vitro growth of HT-29 cells, specifically through the inhibition of the $PGE_2$ production via COX-2.