Quercetin inhibits growth of COLO320 DM cells, derived from a human colon cancer. The inhibitory effect is partially reversible when quercetin is removed from the culture medium. Flow cytometric analysis has revealed that quercetin causes perturbation of the cell cycle, inducing a frozen cell-cycle pattern and a block at the G1/S boundary. The synthesis of a 17-kDa protein was specifically inhibited by the addition of quercetin, and recovered when the cells at the G1/S boundary progressed into S-phase after the removal of quercetin from the culture medium. Furthermore, using synchronized cells obtained by centrifugal elutriation, we have shown that the rate of synthesis of a 17-kDa protein was low in G1, and high in S-phase of the cell cycle. Thus, this protein appears to be cell-cycle-related.