It is well known that interleukin-3 (IL-3) stimulates the proliferation of megakaryocyte colonies. On the other hand, it remains unclear whether IL-3 also promotes the maturation of megakaryocytes. In this report, we investigated the effects of IL-3 on megakaryocyte maturation in vitro using the human megakaryocytic cell line, CMK, which excludes the influence of bone marrow accessory cells. CMK cells were incubated both with and without 2-50 U/ml of recombinant human IL-3 (rhIL-3) and then analyzed for the effects on proliferation, cell size, DNA ploidy and the expression of platelet glycoproteins. We initially confirmed that CMK cells express IL-3 receptors (IL-3R) on the surface, and rhIL-3 stimulates the proliferation of CMK cells, showing their IL-3R function normally. With addition of rhIL-3 to the CMK cell culture system, increments in cell size, a significant shift toward high ploidy classes, and an increased expression of platelet glycoproteins CD29, CD41, CD42a, CD42b and CDw49f were newly observed. These findings suggest that rhIL-3 acts directly on human megakaryocytes and has the ability to directly promote both proliferation and maturation of megakaryocytes, without the help of accessory cells. Furthermore, rhIL-3 induced/enhanced the expression of mRNA for granulocyte-macrophage colony stimulating factor (GM-CSF) and leukemia inhibitory factor (LIF) in CMK cells. Therefore, the effects of rhIL-3 on the maturation of megakaryocytic cell lines demonstrated in this study may be partly mediated by the action of cytokines such as LIF and GM-CSF, which are induced by IL-3 in the megakaryocytic cell line.