<p>Antihuman Mpl antibody is obtained and purified. By using a genetic engineering technique, a single chain antibody of the antihuman Mpl antibody is further constructed. This antibody shows a high agonistic activity. This fact indicates that the activity of an antibody can be reinforced by binding two or more heavy chain variable domains to two or more light chain variable domains with the use of a linker to give a single chain polypeptide.</p>
