Intrinsic optical properties of biological tissue can be modulated with specific genetic alterations, and used as a phenotypic response to probe specific signaling pathways. Utilizing this approach, we used optical scatter imaging to probe the effect of BCL-xL on subcellular particle size distribution within monolayers of living CSM14.1 and iBMK cells. Expression of YFP-Bcl-xL shifted the center of the subcellular particle diameter distribution from 1.5μm to 2μm in CSM 14.1 cells and from 1.5 to 1.8 μm in iBMK cells. This shift was also observed in cells expressing YTP-TM, in which YFP is directly fused to the C-terminal transmembrane (TM) domain of BCL-xL, but not in cells expressing YFP or YFP-BCL-x L-ΔTM, which lack the TM domain. YFP and YFP-BCL-x L-ΔTM were diffusely distributed in the cytoplasm, while YFP-TM and YFP-BCL-xL were localized on the mitochondria. The measured particle sizes, combined with the localization of the TM domain to the mitochondria, suggest that morphological disturbances of the mitochondrial membrane effected by the TM domain of Bcl-xL, underlie the measured optical scatter changes. We have also found that expression of BCL-2, another anti-apoptotic protein, in iBMK cells, results in a subcellular particle diameter increase similar to that induced by BCL-xL. However, BCL-xL-ATM induced as much apoptosis resistance as BCL-xL. Thus, mitochondrial morphology changes are not required for apoptosis resistance. Nonetheless, expression of YFP-TM also conferred a moderate level of apoptosis resistance, while apoptosis resistant iBMK cells lacking Bax and Bak, showed an increase in the light scattering contribution of particles less than 1.5 μm in diameter. Our results suggest a possible secondary role of the BCL-xLTM domain in apoptosis resistance. However, the functional relationship between mitochondrial morphology and apoptosis resistance remains to be fully elucidated.