Anti-human Mpl antibodies were isolated and purified. Anti-human Mpl diabodies and anti-human Mpl sc(Fv)2 were prepared using genetic engineering techniques and anti-human Mpl sc(Fv)2 was also humanized. The diabodies and sc(Fv)2 were assayed for TPO-like agonistic activity, and were found to have activities higher than those of anti-human Mpl antibodies, or activities equivalent to or higher than those of naturally-occurring human TPO ligand.