Abstract: Human monophenotypic cloned cell lines of megakaryocytic lineage are established in vitro through use of adherent stromal cells in long term human bone marrow culture. Long term bone marrow cultures are used for initial adaptation of the cells to culture conditions. Once adapted, cells of the human in vitro cell lines are weaned from the stromal layer until they proliferate in the complete absence of any feeder layer. Seed cells for establishment of human in vitro cell lines were derived from a human solid tumor designated as ATCC CRL 9139 xenograft. Cells of one cloned in vitro cell line designated as CHRF-288-11 exhibits a karyotype and markers characteristic of megakaryocytes and platelets. Cells of the CHRF-288-11 cell line express platelet peroxidate, platelet factor IV, platelet Ca.sup.++ -ATPase, gpIIbIIIa, factor VIII, and MY7, MY9 and HLA-Dr antigens. CHRF-288-11 cell line exhibits a constant karyotype (50, XY).