An ultrastructural study was performed on bone marrow tissue in primary (essential) thrombocythemia to evaluate possible interactions between megakaryocytes and sinusoids. In addition to a preferential localization of megakaryocytes in the subendothelial space, two different kinds of cytoplasmic processes could be discriminated penetrating the sinus wall. Serial sections disclosed that megakaryocytes developed multiple pseudopod-like plump projections derived from their peripheral zone and devoid of organelles. It is tempting to speculate that these ameboid features could serve as anchors to keep the cell in a subendothelial position and to monitor changes occurring in circulation. The second type of cytoplasmic processes reaching into the vascular lumen consisted of tentacle-like elongated protrusions rich in organelles, apparently originating from the intermediate zone. These projections were thought to present either the beginning of megakaryocytes egress into circulation or putative platelets. Frequently, there was an intrasinusoidal localization of megakaryocytes which revealed numerous so-called platelet territories and apparently an enforced platelet shedding. Generally, these features are comparable with aspects obtained from animal studies, following excessively stimulated megakaryo- and thrombocytopoiesis by application of anti-platelet serum.