Recombinant acidic human fibroblast growth factor (aFGF) significantly stimulated the formation of megakaryocyte colonies and the size of MK colonies as well as individual MKs in vitro in mice. When aFGF was combined with recombinant mouse interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-6 (IL-6) at their optimal doses, a synergistic action was found between aFGF and IL-3. The activity of aFGF could be completely abrogated by a monoclonal antimouse IL-6 antibody which specifically neutralized the action of mouse IL-6 but not human IL-6. These data indicate that aFGF provides positive growth signals of megakaryocyte progenitor cells, which can give rise to a synergistic action in the presence of IL-3 and which can be abrogated by the antimouse IL-6 antibody.