Intracellular free calcium (Ca2+ i) is thought to be an important second messenger for phagocyte functions. The fluorescent indicator Quin2 was used to measure and visualize [Ca2+]i in human neutrophils during chemotaxis toward, and phagocytosis of, opsonized zymosan. In neutrophils migrating toward zymosan, [Ca2+]i was highest in the lamellipodium. Neutrophils ingesting opsonized zymosan had the highest [Ca2+]i in the pseudopods and periphagosomal cytoplasm. Most of the increase in [Ca2+]i was from extracellular sources. Regional increments in [Ca2+]i are strategically located to modulate such cellular functions as chemotaxis, oxidative activity, and degranulation.