Proliferation of CD34⁺ cells purified by negative depletion of lineage-positive cells (n = 5) and cultured in serum-free medium in the presence of MIP-1α, IL-3, IL-6, IL-11, FL, SCF, and MGDF. As shown by the increase of CD34⁺/CD61⁺ cells and of CFU-mega, expansion of megakaryocytic progenitors peaked on day 7. Expansion of CD61⁺ cells was particularly evident between days 5 and 7 and reached a plateau in the second week of culture, when the number of clonogeneic cells declined. In fact, cell differentiation was shown by the striking decrease of the CD34 phenotype, whereas 30% to 50% of cells maintained the CD61⁺ phenotype.