Average platelet size, platelet count, and ³⁵S‐incorporation into platelets were compared as methods for the measurement of thrombopoietin‐stimulated thrombopoiesis. In mice injected with rabbit anti‐mouse platelet serum (RAMPS) average platelet size was shown to be increased as mice were recovering from thrombocytopenia. Also, ³⁵S‐measurements on platelets of these mice showed significant increases in cpm/average platelet 2–4 days after RAMPS treatment. Significant increases in ³⁵S‐incorporation into the total circulating mass of platelets were found on days 3–4. In normal mice or mice in rebound‐thrombocytosis injected with thrombopoietin, platelet size remained unchanged, whereas the platelet count and ³⁵S‐incorporation into platelets were shown to be significantly increased. Moreover, a dose‐response experiment in mice pretreated with RAMPS showed a slight increase in platelet count as the dose of TSF was increased, but platelet sizes were unaltered. The %³⁵S‐incorporation into platelets showed a significant linear dose‐response, i.e. as the dose of thrombopoietin was increased, an increase in %³⁵S‐incorporation into platelets was observed. These data indicated that of the three indirect measurements of thrombopoietin, the %³⁵S‐incorporation into mouse platelets was the most sensitive, followed by platelet counting; the least sensitive measurement of thrombopoiesis was change in platelet size.