It has been proposed that Ca2+ activation of calpain-1 is important for the rapid cell shape changes which accompany phagocytosis. In this paper, we use a fluorogenic calpain substrate, (CBZ-Ala Ala)(2) R110, and find that there was a low calpain activity measureable in resting (ie without intentional activation) neutrophils, but that it was accelerated by an elevation of cytosolic free Ca2+ (ionomycin -induced) and inhibited by calpeptin (an established calpain-1 inhibitor). The fluorescence signal was sufficiently bright for detection in individual neutrophils that enabled the quantification of dynamic changes in calpain activity to be related to elevations in cytosolic Ca2+ within individual neutrophils. It was found that during phagocytosis of C3bi-opsonised zymosan particles, calpain activity was elevated incrementally, each step increase corresponding to the phagocytosis of an individual particle. The sub-cellular source of the fluorescent product of calpain activity was the phagocytic site itself and originated at the phagocytic cup. It was thus concluded that calpain was activated locally during the formation of the phagocytic cup. These data were consistent with central role of Ca2+ activated calpain activation in controlling phagocytosis. (C) 2019 Elsevier Inc. All rights reserved.