The type of cell death occurring in the same cell line (MG-63 human osteosarcoma cells) grown in monolayer or as three-dimensional spheroids after exposure to 5 Gy of ionizing radiation was determined. Morphological analyses using the chromatin dye Hoechst 33258 demonstrated that spheroids showed the typical characteristics of apoptosis, while monolayer cells revealed those typical of mitotic catastrophe. In order to better characterize these two types of cell death, the role of caspases was examined in irradiated monolayer cells and spheroids using the broad spectrum caspase inhibitor zVAD-fmk. Death in monolayer cells was caspase-independent, whereas spheroid death was characterized by caspase dependence. Members of the Bcl-2 family of proteins and survivin involved in cell death processes were also studied by Western blot analysis. The pro-apoptotic protein Bax increased in spheroids, whereas this protein remained unchanged in monolayer cells after the same 5-Gy irradiation. The anti-apoptotic protein Bcl-2, on the other hand, remained unchanged in both monolayer cells and spheroids. Finally, survivin increased significantly after irradiation in both cells in monolayer and spheroids. The results presented suggest that three-dimensional cell organization leads to a different type of cell death after exposure to ionizing radiation. Thus, the use of spheroids, a cell model which mimics in vivo solid tumors more closely than cells grown in monolayer, is more appropriate when investigating the effects of antineoplastic treatments such as ionizing radiation.