This invention provides an agonist antibody to a human thrombopoietin receptor (alias: human c-Mpl). More particularly, this invention provides an agonist antibody to a human thrombopoietin receptor, wherein the agonist antibody comprises: antibody constant regions comprising (1) amino acid sequences in a heavy chain constant region and a light chain constant region of a human antibody, (2) an amino acid sequence of a heavy chain constant region with a domain substituted between human antibody subclasses, and an amino acid sequence of a light chain constant region of a human antibody, or (3) amino acid sequences comprising a deletion(s), substitution(s), addition(s), or insertion(s) of one or several amino acid residues in the amino acid sequences of (1) or (2) above and antibody variable regions capable of binding to and activating a human thrombopoietin receptor and wherein the agonist antibody has the properties: (a) that the antibody induces colony formation at a concentration of 10,000 ng/ml or lower as determined by the CFU-MK colony formation assay using human umbilical-cord-blood-derived CD34+ cells and (b) that the antibody has a maximal activity at least 50% higher than that of PEG-rHuMGDF and an 50% effective concentration (EC50) of 100 nM or less in the cell proliferation assay using UT7/TPO cell. Also provided is a pharmaceutical composition for treating thrombocytopenia comprising said antibody.
