In studying some of the properties of collagen responsible for the ability to aggregate platelets it was found that thermal treatment at pH 2.5 of acid-soluble human collagen resulted in a sharp reduction in relative viscosity and platelet aggregating activity at about 35 degrees C. The reduction in viscosity is known to be associated with structural transition from triple helical to random coil form and it is postulated that the native structure of collagen is essential for its platelet aggregation effect. Blockage of the free amino groups by deamination, N-acetylation, or treatment with dinitrofluorobenzene resulted in over 90% reduction in platelet aggregating activity. Addition of cationic proteins to collagen, removal of the negatively charged telopeptides by treatment with pepsin, or acetylation of the free carboxyl groups did not significantly affect the platelet aggregating activity of collagen. On the basis of these findings it is suggested that the free amino groups and specifically the epsilon amino groups of lysine are critical for the platelet aggregating activity of collagen whereas the carboxyl groups are of relatively little importance.