Micronuclei (MN) formation was defined as a form of radiation-induced damage in MCF-7 cells. MN appeared post-mitosis and were scored in bi-nucleated cells of cytochalasin B treated cultures. MN were surrounded by an envelope composed of inner and outer membranes, and contained fragmented chromosomes. However, typical features of apoptosis, such as chromatin margination or condensation were not observed. Reducing serum concentration resulted in a decreased MN formation, suggesting that serum factors directly affected MN formation and/or that serum depletion decreased the availability of radiation sensitive MN-forming cells for mitosis. Irradiation of G1 and S phase enriched populations revealed that S phase cells were more prone to MN formation than G1 cells. Radiation-induced chromosomal aberration can therefore be modulated by altering serum level and cell cycle distribution.