Abstract: AIM: To study the mechanism of inhibitory action of howiinol A(GHM-10) on synthesis of DNA in cancer cells. METHODS: Single cell gel electrophoresis was used to detect the DNA molecular damage induced by GHM-10 in L1210 cells. Alkaline elution technique was used to investigate the effect of GHM-10 on the length of DNA strands of L1210 cells. Dehelix effect of GHM-10 on superhelix pUC 18 DNA was used to estimate its influence on the activity of DNA topoisomerase II. RESULTS: After L1210 cells were treated with 4 and 10 μg.ml^-1 of GHM-10 for 4.5 h, comet like tails could be seen in cells under a fluorescence microscope after electrophoresis, indicating that DNA damage was induced by GHM-10. DNA single strand breaks were demonstrated when L1210 cells were treated with GHM-10 in concentrations of 4 and 25 μg.ml^-1 for 5 h. When L1210 cells or protein isolated from L1210 cells were treated with GHM-10, the activity of DNA topoisomerase II was inhibited. CONCLUSION: GHM-10 induces damage to DNA molecules in L1210 cells and inhibits the activity of topoisomerase II both inside and outside the cells.