Disclosed are transgenic non-human mammals, which useful for the screening of thrombopoietin mimetics, thrombopoietin receptor agonists, or thrombopoietin receptor antagonists active on the human thrombopoietin receptor. The transgenic non-human mammal has a genome that comprises a stably integrated transgene construct comprising a polynucleotide sequence encoding a humanized thrombopoietin receptor wherein said transgenic non-human mammal has a baseline blood platelet count corresponding to a physiological blood platelet count of a matched non-transgenic non-human mammal. The chimeric thrombopoietin receptor comprises either the transmembrane domain of a human thrombopoietin receptor or both the extracellular and transmembrane domains of a human thrombopoietin receptor operably coupled to a cytoplasmic domain of a non-human thrombopoietin receptor.
