Fluorescence lifetime images of HeLa cells expressing enhanced green fluorescent protein (EGFP) have been measured as apoptosis is induced by tumor necrosis factor-alpha (TNF-alpha) in combination with cycloheximide. The fluorescence lifetime of EGFP is found to decrease after the induction of apoptosis, indicating that the change in environment occurs around the chromophore of EGFP with the apoptosis process. The fluorescence lifetime imaging technique can be used to perform in vivo observation of cell death processes. Fluorescence lifetime measurements are useful to examine the induction of the apoptosis process, even when a morphological change of each cell cannot be observed because of a low spatial resolution.