Guinea pig megakaryocytes have been isolated from bone marrow by a combination of density equilibrium and velocity sedimentation techniques. The resulting cell suspension is approximately 20-30% megakaryocytes. The affinity of cell surface membranes for tannic acid staining in the presence of calcium has been demonstrated. Morphologic aspects of platelet formation is being studied. The morphologic effects of certain antimitotic agents is being currently described. The effect of certain agents known to degranulate platelets is currently under investigation. Planned studies included: Those on cell maturation with light microscopic phase, Nomarski, electron microscopic techniques. Aspects on cell maturation, both nuclear and cytoplasmic, will be studied by microspectrophotometric histochemical and isotope incorporation techniques.