The present studies have investigated, for the first time, the synergistic effects of interleukin-4 (IL-4) and IL-11 on the growth of single murine bone marrow progenitor cells. These studies suggest that IL-4 and IL-11 are synergistic hematopoietic growth factors, enhancing colony formation of bone marrow progenitors from normal mice in the presence of colony-stimulating factors or stem cell factor, whereas neither IL-4 nor IL-11, alone or in combination, resulted in colony formation. However, in the presence of a neutralizing anti-TGF-beta antibody, IL-11 plus IL-4 induced clonal growth of primitive Lin-Sca1+ progenitors. Furthermore, here we report several observations extending the knowledge about IL-4 and IL-11 as synergistic factors. In addition to the established ability of IL-11 to enhance IL-3- and GM-CSF-induced colony formation, IL-11 also enhanced the number of G-CSF- and CSF-1-stimulated colonies of mature (Lin-) and primitive (Lin-Sca-1+) hematopoietic progenitors cultured at the single-cell level. In contrast, IL-4 bifunctionally regulated the growth of Lin- progenitors, whereas the growth of single Lin-Sca=1+ progenitors was unaffected or enhanced in the presence of IL-4. Finally, IL-4 and IL-11, in combination, potently synergized to enhance the high-proliferative-potential colony-forming cell colony formation of Lin-Sca-1+ progenitors in response to all four CSFs and to SCF.