The R10-G<sub>i</sub> probe exhibited an increased FRET signal in accord with the LTB<sub>4</sub>-dependent activation of G<sub>i</sub> Furthermore, neutrophil-like differentiated human leukemia cell line 60 that expressed the intrinsic BLT1 displayed temporal G<sub>i</sub>-protein activation in an area localized to the leading edge during chemotaxis in a shallow gradient of LTB<sub>4</sub> These findings afford an opportunity to clarify the mechanisms underlying the subcellular regulation of G<sub>i</sub>-protein activity, as well as GPCR-mediated ligand sensing, during chemotaxis in mammalian cells.-Masuda, K., Kitakami, J., Kozasa, T., Kodama, T., Ihara, S., Hamakubo, T. Visualization of ligand-induced G<sub>i</sub>-protein activation in chemotaxing cells.