Effects of recombinant human interleukin (IL)-13 on in vitro haemopoiesis from non-adherent mononuclear cells (NAMC) or highly enriched CD34+ cells of human cord blood (CB) were studied. IL-13 significantly increased megakaryocyte (MK) colony formation from either NAMC or CD34+ cells cultured in a plasma clot system supplemented with aplastic anaemia serum (AAS) and phytohaemagglutinin-stimulated human peripheral blood leucocyte-conditioned medium (PHA-LCM) in a dose-dependent manner. Experiments using a modified plasma clot culture, in which normal AB serum and various cytokines were added to replace AAS and PHA-LCM, demonstrated an increased MK colony number in the presence of IL-13, especially in combination with IL-3. However, IL-13 had no stimulatory effect, but rather a slight inhibitory effect in some cases on granulocyte-macrophage (GM) colony formation in both plasma clot cultures. Furthermore, the growth of GM progenitor cells in a methylcellulose culture system in the presence of IL-3, GM-CSF, Epo, G-CSF or in combination was significantly inhibited by the addition of IL-13. On the other hand, high concentrations (100 ng/ml) of IL-13 were needed to cause a slight inhibition on the growth of BFU-E-derived colonies under the same methylcellulose culture. These results indicate that IL-13, alone and synergistically with the effect of IL-3, promotes MK colony formation, but it inhibits the growth of GM and erythroid progenitor cells in vitro.