Stimulation of Platelet Production by Serum of Platelet-Depleted Rats

Four subcutaneous injections, given during a 2-day period, of serum from platelet-depleted rats into untreated or splenectomized rats caused an increase in the number of circulating platelets to as much as 167% of the initial count. The rise in peripheral platelet count began 2 days after the first injection, reached a peak at 5 days, then declined to control values at 8 days. Equal amounts of serum from untreated donors did not produce this response. The results indicate that the active serum contains an agent (thrombopoietin) that stimulates production of blood platelets.