Previous studies on the platelet agonist activity of condensed cotton bracts tannin have used tannins with an apparent molecular weight greater than 10,000 daltons. To determine if platelet agonist activity was related to polymer length, cotton mill dust was fractionated by Amicon ultrafiltration and analyzed for platelet agonist activity. All tannins greater than 1,000 daltons were equally potent as platelet agonists while those smaller than 1,000 daltons had no agonist activity. This result was confirmed with two synthetic procyanidin dimers which had little or no activity. In addition, methylation of the tannin hydroxyl groups resulted in the loss of platelet agonist activity. Thus, the platelet agonist activity of tannin requires a minimum polymer length and the presence of free hydroxyl groups.