The effects of tetrandrine, a benzylisoquinoline alkaloid useful in the treatment of silicosis, on a broad range of human neutrophil activities was examined in vitro. Random movement, chemotaxis and phagocytosis were significantly suppressed. There was minimal but significant inhibition of lysosomal enzyme secretion from specific (secondary) but not azurophil (primary) granules. The same concentration of tetrandrine (10 micrograms/ml) caused marked depression of hexose-monophosphate shunt activity and hydrogen peroxide production, but inhibition of superoxide anion generation was observed even at a concentration of 0.1 microgram/ml. This discrepancy was attributed to the capacity of tetrandrine to scavenge oxygen radicals, as shown by experiments using hypoxanthine-xanthine oxidase to generate superoxide. These potent antiphagocytic and antioxidant properties of tetrandrine may account for some of its remarkable anti-inflammatory effects.