The concept that the demarcation membrane system delineates platelets within the cytoplasm of megakaryocytes has been examined. In short-term culture of mouse bone marrow, mature megakaryocytes extended long, attenuated processes that were found by electron microscopy to have a limited amount of invaginated membrane. When such megakaryocytes were exposed to microtubule depolymerizing agents, the attenuated processes retracted, became thicker, and an extensive demarcation membrane reappeared. It is suggested from the results that the demarcation membrane system functions to provide a membrane reserve that undergoes evagination during the formation of attenuated processes and thereby envelops putative platelets, rather than to demarcate platelets in the maturing megakaryocyte. The term “invaginated membrane system” is considered more appropriate than “demarcation membrane system.”