Tannin-mediated release of radiolabelled arachidonic acid from rabbit alveolar macrophages was found to require the presence of extracellular calcium. The extent of radiolabel release to a standard tannin challenge was directly related to extracellular calcium concentrations up to 3.5 mM. In order to investigate the role of calcium in the mechanism of tannin stimulation, we used the fluorescent calcium indicator Indo-1. Tannin was found to evoke a rapid increase in the intracellular calcium concentration of macrophages when examined by flow-cytometry. The source of calcium increase appeared to be predominantly extracellular rather than a release from intracellular stores. The data suggest that tannin induces an influx of calcium across the plasma membrane which may trigger a cell-signalling pathway resulting in the accumulation of free arachidonic acid.