Hematopoietic lineage-restricted stem cell growth has been shown to be significantly inhibited by the addition of exogenous transforming growth factor-beta (TGF-beta) to Dexter-type long-term murine bone-marrow cultures. In order to examine whether TGF-beta produced by these cells has a role in hematopoietic growth regulation, Dexter cultures have been treated with either 1D11.16, a monoclonal antibody that neutralizes the biological activity of TGF-beta types 1, 2, and 3, or with a control antibody. The composition and cellularity of the nonadherent cell populations in these cultures were assessed weekly. Treatment with anti-TGF-beta antibody resulted in a five- to 20-fold increase in nonadherent cells in the cultures when compared to either the control or untreated cultures by week 4. The majority of these cells were granulocyte/macrophage-lineage cells as assessed by histologic and flow-cytometric analysis. There was also a significant increase of megakaryocytes in cultures treated with anti-TGF-beta antibody. Stem-cell analysis, using a colony-forming unit-spleen (CFU-S) assay that combined both the adherent and nonadherent populations from either 4- or 6-week cultures, showed that there are an equivalent number of hematopoietic stem cells per 10(6) cells regardless of antibody treatment. Therefore, cultures treated with anti-TGF-beta antibody contained at least three times as many stem cells as the control cultures. Finally, kinetics studies show that the presence of anti-TGF-beta antibody is required from the onset of culture to produce these effects. These results suggest that TGF-beta is involved in normal growth regulation of bone-marrow hematopoietic cells. By addition of a neutralizing antibody, the normal TGF-beta negative growth signal is disrupted, allowing for expanded growth of several cell populations.