Summary.The effect of human recombinant erythropoietin (EP) on the growth of human marrow megakaryocyte colony forming units (CFU‐M)in vitrowas investigated by the use of a plasma clot assay. EP as a single stimulating factor or as an additional factor to optimal concentration of leucocyte conditioned medium (PHA‐LCM) had no effect on the number of CFU‐M derived colonies. However, addition of EP (O·5‐1 U/ml) to cultures with suboptimal concentrations of PHA‐LCM increased megakaryocyte colony formation by 50‐90 but had no effect on the number of granulocytic‐monocytic colonies (CFU‐GM). Exposure of marrow cells to EP for 24‐48 h in liquid suspension cultures, followed by removal of the hormone and assaying the cells for CFU‐M in plasma clots, resulted in a 50‐100 increase of megakaryocyte colony formationin vitro.The augmenting effect of EP on CFU‐M growthin vitrowas abolished when EP was added to the medium after the third day of culture. The presence of factors in human serum and in PHA‐LCM was an absolute requirement for the hormone to exert its potentiating effect on human CFU‐M growthin vitro.Recombinant EP potentiates the growth of human marrow CFU‐M and this effect seems to be exerted during the early stage
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