Cotton bracts tannin and immunoglobulin C (IgG) act synergistically at low tannin concentrations to enhance the release of 5-hDroxytryptamine (5-HT) from human platelets. Augmentation of this reaction by the endotoxin from the rough mutant S. Minnesota R595 suggests that the enhanced release is due to the formation of tannin-IgG aggregates which function as platelet secretogogues.