Tension testing of wet bovine haversian cortical bone demonstrated marked plastic behavior. Progressive surface decalcification of this bone with dilute hydrochloric acid resulted in progressive decreases in the tension yield point and the ultimate stress with no change in the yield strain or ultimate strain unless decalcification was complete. The slope of the plastic region remained identical throughout decalcification. These findings are consistent with an elastic-perfectly plastic model for the mineral phase of bone tissue in which the mineral contributes the major portion of the tension yield strength. The slope or stiffness of the plastic region of the stress-strain curve is a function only of the properties of collagen, which itself plays a minor role in the tension yield strength of bone.