This invention provides an agonist antibody to a human thrombopoietin receptor (human c-Mpl), and pharmaceutical compositions comprising the same for use in treatment of thrombocytopenia. The disclosed agonist antibody comprises (1) antibody constant regions comprising heavy and light chain constant regions, each of which may optionally contain domain substitutions, or may contain deletions, substitutions, additions, or insertions of amino acid residues, and (2) antibody variable regions capable of binding to and activating a human thrombopoietin receptor. The-agonist antibody further induces colony formation at a concentration of 10,000 ng/ml or lower, and has a maximal activity at least 50% higher than that of PEG-rHuMGDF and an 50% effective concentration (EC50) of 100 nM or less.