PURPOSE: To obtain the titled acid dispersion liquid having high activity and suitable as a clinical examination reagent, especially a reagent for the determination of the platelet coagulation activity and platelet adhesion activity, by treating the Achilles tendon of an animal with a protein hydrolase, followed by other specific treatments.

CONSTITUTION: The Achilles tendon of an animal is treated with a protein hydrolase free from collagenase. The treated product is washed with a aqueous solution of an inorganic salt, dried, and pulverized. The obtained fine powder of collagen fiber is swollen by immersing in an acid, and the swollen fiber is further pulverized to obtain the objective acid dispersion of collagen fiber.