Evidence has been sought for megakaryocyte maturation in long-term cultures of mouse bone marrow. Cultures up to 14 weeks of age were examined for the presence of megakaryocytes with processes, that is, resembling the morphological appearance seen in vivo prior to platelet liberation. Such cells were found floating just above the adherent stromal layer using low magnification phase contrast microscopy. It was rare to observe as many as 20 of these cells per 25-cm2 flask. At higher magnification, processes were seen to be attenuated with constrictions at intervals along their length. Time-lapse photography was used to follow the development and behavior of the processes. Direct evidence of rupture was very rare; generally the megakaryocytes retracted their processes within 48 h. Careful searching of cultures occasionally revealed the presence of several process fragments, and sometimes individual platelets were found. Ultrastructurally, the processes were seen to contain organelles that are usually associated with platelets. The observations applied to both Dexter and Whitlock-Witte cultures. It is concluded that maturation of megakaryocytes occurs in long-term marrow culture to the point where platelet release appears imminent. Final rupture is rare and may require shearing forces, which in vivo would be provided by blood flow.