Some environmental mutagenic agents cause genomic instability and increase susceptibility of DNA damage. One of them is mitomycin C which is connected to DNA as an alkylating factor and affects susceptible cells to reduction reactions. This drug is used in chemotherapy and treatment of tumors. Study of genomic instability in the presence of different concentrations of MMC can show susceptibility of DNA damage in the patients who are under chemotherapy with this drug. For this purpose, SCE is a qualified method that shows the number of sister chromatid exchanges in the metaphasic chromosomes. The number of 105 lymphocytic cells which were separated with ficol, were cultured in media (5ml, F12 15%-20%FCS) that contains mitogen of PHA (phytohemagglutinin) and MMC in the concentrations of 3ng/ml, 6ng/ml and 9ng/ml and a control sample without MMC. The specific concentration of BrdU was added after 24 hours to cell cultures. Then metaphasic cells were halted in the metaphasic stage with colchicine after 48 hours and were stained with SCE method and were studied for the number of sister chromatid exchanges in each metaphasic plaques. Evaluation of 100 metaphasic plates showed that SCE was %3.35 in the control cells while it was %5.43, %7.1 and %8.13 in the treated cells with MMC in the concentrations of 3ng/ml, 6ng/ml and 9ng/ml. In view of the results, it is clear that MMC can cause genomic instability even in the low concentrations and it can increase SCE so that the level of SCE is become the most with the concentration of 9ng/ml and the least with the concentration of 3ng/ml. In view of relation between SCE and DNA damage, we can conclude that the genome of normal cells will be damaged in the presence of MMC and in the patients who are under chemotherapy with this drug. It means that the genome of cells will become sensitive to mutation in the presence of low concentrations of MMC. Therefore we can postulate that we should use the concentrations of less than 3 ng/ml in order to decrease mutagenic effects of MMC in normal cells.