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To study the role of cell death in the aging process, cell death during spontaneous cellular senescence in vitro was examined with normal human fibroblasts. A small subset of the senescent cells showed aberrant morphology such as remarkable nuclear fragmentation or multiple micronuclei, and such cells often showed positive reactions with antibody to phosphorylated pRb. Cells showing caspase activation and binding of Annexin V, which indicate apoptotic change, increased in the senescent phase in flow cytometry analysis. Propidium iodide–positive cells, however, also increased with passaging. The results suggest that both apoptosis and necrosis are involved in cell death of senescent human fibroblasts.