Intravenous administration of huMkMPs into untreated WT mice increases murine platelet concentrations and ameliorates thrombocytopenia (TP) in thrombocytopenic mice 16 hours after administration. TP was induced by intraperitoneal administration of anti-CD41 antibody. After 8 hours, 2 × 106 huMkMPs were administered intravenously into untreated or thrombocytopenic mice via the tail vein. (A) Murine platelet levels were measured 24 hours after antibody injection (16 hours after huMkMP administration) of untreated mice (n = 7), mice treated with huMkMPs (n = 5), thrombocytopenic mice (n = 9), or thrombocytopenic mice treated with MkMPs (n = 8). (B) The number of reticulated (newly synthesized) platelets was measured 24 hours after antibody injection (16 hours after huMkMP administration) by flow cytometry for a subset of mice in the 4 murine cohorts in panel A. The data are presented as the percentage of the total platelet count. Error bars represent the SEM. *P < .05; **P < .01; ***P < .001.