Thrombopoietin (TPO) is a major hematopoietic growth factor which regulates the production of megakaryocytes and platelets. Recent cloning of its c-DNA and availability of recombinant TPO have enabled us to examine the biological effects of TPO in various systems, including human platelets. In this review, we briefly describe the recent progresses made in the studies of TPO and its cognate receptor, c-Mp1.