To investigate effect of poly (ADP-ribose) polymerase inhibition on the proliferation of CT26 cells in vitro and the mechanism of this effect. CT26 cells were treated with a range of concentrations of 5-Aminoisoquinolin-1-one (PARP inhibitor) in vitro. MTT assays and flow cytometry were used to determine the proliferation and cell cycle distribution, respectively, of the cells. The expression of PARP-1 was investigated by Western blot. The binding of Nuclear Factor-kappaB to DNA was detected by electrophoretic mobility shift assay. The proliferation of CT26 cells was significantly inhibited by 5-AIQ induced PARP inhibition in a dose-dependent manner. Proliferation was inhibited by 17.5% at 100 microM 5-AIQ, by 27.6% at 500 microM 5-AIQ and by 39.9% at 1000 microM (P < 0.05). After treatment with 5-AIQ, the proportion of cells in G(0)/G(l) phases increased and the proportion of cells in S phase decreased. The expression of PARP-1 was attenuated in 5-AIQ-treated CT26 cells (P < 0.05) and the binding of NF-kappaB to DNA binding was similarly diminished (P < 0.05). These results suggest that PARP inhibition reduced the proliferation of CT26 cells in vitro and influences the cell cycle. This inhibition is mediated by inhibiting PARP-1, which then diminishes the activity of NF-kappaB.