We have developed a unique in vitro model that promotes differentiation of megakaryocytes into platelets. When megakaryocytes isolated from guinea pig bone marrow were cultured on hydrated rat tail collagen gels, cells spontaneously formed elongated, beaded processes that fragmented to yield cytoplasmic pieces with the same size and internal composition as individual platelets. Addition of nocodazole at the initiation of cultures blocked process formation, while addition of nocodazole to cells with previously established processes resulted in their retraction. The addition of taxol to cultures resulted in abnormally thick processes that were tightly adherent to the underlying substratum, and did not bead or fragment. Cytochalasin D accelerated process formation and fragmentation of megakaryocytes cultured on collagen gels by twofold. On the basis of these results, we propose a model for platelet formation in culture that involves the following steps: adherence of megakaryocytes to the underlying extracellular matrix; dilation of the demarcation membrane system and breakdown of the actin-rich peripheral zone; microtubule-based extension of pseudopodia, which are no longer adherent to the substratum; and fragmentation into platelets by the coalescence and fusion of demarcation membrane vesicles with the plasma membrane. We feel that this distinctive culture system closely approximates thrombocytopoiesis in vivo, thus allowing detailed elucidation of this important process.