Summary. Megakaryocyte (MK) maturation culminates in release of blood platelets through proplatelet extensions. MKs presumably delay elaborating proplatelets until synthesis of platelet constituents is complete. Recent insights from investigation of a classic human congenital macrothrombocytopenia, the May–Hegglin anomaly, and related MYH9-associated disorders shed new light on underlying mechanisms. The findings reviewed in this article implicate myosin IIA, the non-muscle myosin heavy chain product of the MYH9 gene, in restraining proplatelet formation until MKs achieve terminal maturity. Loss of myosin IIA function, through dominant inhibitory mutations in humans, targeted gene disruption in mice, or manipulation of cultured MKs, seems to accelerate proplatelet formation. The resulting process is inefficient and produces platelets that vary widely in size, shape and content. Several lines of evidence suggest that the Rho-ROCK-myosin light chain pathway restrains proplatelet formation through myosin IIA. These findings illustrate that mammalian thrombopoiesis is complex and subject to both positive and negative regulation.