Condensed tannins, present in the dust generated from the processing of woody plants, are potent biological agents. They modulate the function of a variety of cell types including alveolar macrophages, peripheral blood monocytes, platelets and T-lymphocytes, vascular endothelial cells and airway epithelial cells. Several different protocols have been used to isolate the condensed tannins. Standard spectrophotometric methods exist to assess the mass purity of the isolated material. However little or no attention has been paid to developing a standard assay to assess the biological activity. Here, we present studies to support a proposal that secretion of 5-HT from human platelets is a good assay for the biological activity of tannin. Two advantages of the assay are that it is not sensitive to the number of platelets used and is relatively insensitive to the time of incubation. Similar tannin dose response curves for 5-HT secretion have been obtained with 8 tannin preparations isolated over a five year period by two different methods.