522 patients with chronic thrombocytopenia were studied over a period of three years. 252 of them suffered from idiopathic, acquired, thrombocytopenic purpura with excessive platelet destruction (platelet count less than 50,000 and mean survival less than 3 days). All of these cases were studied by using Indium oxinate labelled autologous platelets. The essential finding in these cases was the absent or only slight reactive hyperproduction, regardless of age and regardless of the severity of platelet destruction. No significant modification in platelet production was observed in the presence or in the absence of anti-platelet antibodies. No significant difference in the haematopoietic response, which was only exceptionally greater than twice the normal value, was observed whether thrombocytopenia was or not very long-standing, very severe, or affected a child or an adult. Megakaryocytosis, assessed on smears or biopsy sections, and the mean platelet volume do not constitute parameters for measuring platelet production. These findings confirm previous findings reported in the 1970s using radioactive chromium as the tracer. They demonstrate that the regulation of megakaryocyte production differs from that of erythropoietic production. They also show that determination of platelet production based on the survival of autologous platelets is able to distinguish between excessive destruction, even in the absence of a haematopoietic response, and defective production, demonstrated when the haematopoietic response is less than 50% of the normal value.