Murine megakaryocyte precursor cells giving rise to platelet-shedding thrombocytes will be quantitated and characterized, and factors (thrombopoietin and feedback inhibitors) regulating these cell populations will be investigated using short-term (semi-solid agar) and long-term (liquid suspension) tissue culture systems. The semi-solid agar culture assay allows precursor cells (1) to be quantitated; (2) to determine their cell cycle characteristics; (3) to assess direct stimulators and potentiators of thrombopoiesis; (4) to investigate clonal variation in ploidy values and platelet production from single precursor cells, and (5) to examine the relative role of stimulators in megakaryocyte formation and platelet shedding. The long-term culture procedure allows a kinetic approach in a system in which trafficking is not a problem. Regulatory processes can be analyzed by the addition of candidate factors either to induce a wave of megakaryopoiesis or to modulate continuing thrombopoiesis or to shut down differentiation.