The liquid culture system with platelet production was used to investigate the effects of thrombocytopoiesis-stimulating factor (TSF) on terminal cytoplasmic maturation of megakaryocyte and thrombopoiesis. TSF, at concentrations from 0.5-2.0 U/ml, stimulated megakaryocyte DNA synthesis, cytoplasmic maturation and isotopic incorporation into platelets, whereas megakaryocyte and platelet count remained unchanged. Our data indicate that TSF, stimulates platelet production by increasing the size of platelet.