The effect of tannin on three macrophage functions (spreading, phagocytosis and arachidonic acid release) was examined. The ability of rabbit alveolar macrophages to spread on an adherent surface was inhibited by tannin in a dose dependent mariner. Maximal inhibition occurred at 12.5 pg tannin/mL and higher concentrations. In contrast, the effect of tannin on the phagocytosis of unopsonized latex beads was bi-phasic. At low concentrations (<12.5 pg/mL), phagocytosis was enhanced while higher tannin concentrations inhibited phagocytosis. Finally, tannin at concentrations that fully inhibited both spreading an d phagocytosis promoted the release of arachidonic acid from the cell. These modifications in alveolar macrophage function could alter their ability to deal effectively with the inspired dust load and thereby contribute to the development of byssinosis.