Thrombopoietin, the ligand for the c-mpl receptor, promotes proliferation and maturation of megakaryocytes. An ELISA using a chimaeric receptor, mpl-IgG, for capture, and rabbit antibody to thrombopoietin for detection was developed for the quantitation of thrombopoietin in human serum or plasma. This ELISA preferentially detects full-length thrombopoietin compared to the bioactive N-terminal half of the molecule which has homology to erythropoietin. Thrombopoietin was not detected (<0.16 ng/ml) in 88/89 healthy individuals. However, elevated thrombopoietin concentrations of up to 3 ng/ml were detected in 59/63 thrombocytopenic patients, including cancer patients following chemotherapy. In cancer patients receiving chemotherapy with (n = 12) or without (n = 6) peripheral blood progenitor cell transplantation, thrombopoietin concentrations varied inversely with platelet counts throughout the treatment period. In general, patients who received myeloablative chemotherapy on days −7 to −2 and peripheral blood progenitor cell transplantation on day 0 had high thrombopoietin levels (0.6–2.9 ng/ml) around day 5. Low platelet counts (<20 × 109/l) occurred between days 4 and 9. Patients who received high-dose chemotherapy on day 1 (equivalent to day −7 for transplantation patients) to day 6 without transplantation had high thrombopoietin concentrations (1.4–2.3 ng/ml) around day 13 and low platelet counts occurred between days 7 and 17.