Cancer, as one of the most important concerns of human health, is an illness with geneticcause. There are various theories to explain how it originates. Genetic and phenotypicinstability are two known features of cancer cells, but their cause is not clear. Geneticinstability can be brought about in two different levels. In a small number of tumor cases thisinstability is noticed at nucleotide level. While in many cancer cases the instability observedis at chromosome level, consisting of loss or gain of complete chromosomes (aneuploidy)and structural anomalies. Identifying the reason for the occurrence of such instabilities andthe understanding of the importance of each one of those chromosomal incidence, can opennew doors in coping with this problem. For this reason the present study was carried out withthe purpose of studying the influences of gamma radiation, as one of the known clastogenicfactors, on vinblastine-induced aneuploid L929 cells. The L929 cells were treated with threedoses(0.5, 1.5 and 2 ng/ml) of vinblastine for 24 hours. Cells treated with 0.5 ng/ml ofvinblastine were harvested 24, 48 and 72 hours post treatment. The cells were exposed to1Gy of gamma radiation after 72 hours of vinblastine treatment. In all part of the experimentcells were harvested 24 hours after cytochalasin B treatment. Induced-chromosomaldamages were investigated using micronucleus assay in binucleated cells. The frequency ofmicronuclei was calculated in the harvested cells. The best concentration for vinblastine wasspecified to be 0.5 ng/ml. It was observed that 72 hours after treatment with vinblastine thefrequency of micronuclei decreased to the base line. Gamma-irradiation of the cells treatedwith 0.5 ng/ml of vinblastine and recovered 72 hours post treatment, revealed a significantdecrease in the frequency of micronuclei in comparison to the cells exposed only to gammaradiation. The results indicate that the excessive occurrence of aneuploidy among cells, as aresult of treatment with vinblastine, an aneugen factor, has not predisposed them toclastogenic damages caused by gamma radiation. This issue highlights the important andindependent role of aneuploidy in causing cancer, as compared to genetic mutations orchromosomal damages. The results also indicate that treatment of L929 cells with vinblastinehad a protective effect against structural damages caused by gamma radiation.