Summary
Quantitation of megathrombocyte (large platelet) number was used as an assay for the detection of a humorally-transmitted thrombopoietic stimulus, thrombopoietin. Donor guinea pigs were depleted of circulating platelets by the injection of rabbit anti-guinea pig platelet antibody. Plasma from these donor guinea pigs, when injected into recipient guinea pigs raised their platelet count 1.5 fold and their megathrombocyte number 2.7 fold when compared to plasma injected from donor guinea pigs with normal platelet counts. A lag period of 4 to 5 days preceded the rise in platelet count and megathrombocyte number.