We previously showed that quercetin (3,3′,4′,5,7 pentahydroxyflavone) inhibits in a dose-dependent manner the growth of acute leukemias and is able to enhance the antiproliferative activity of cytosine arabinoside. We show here that quercetin inhibits the clonogenic activity of 20 of 22 acute leukemias (AL; 4 M1-AML, 3 M2-AML, 2 M3-AML, 3 M4-AML, 3 M5-AML, and 7 ALL). In the present report, we show that the induction of transforming growth factor-beta 1 (TGF-beta 1) in leukemic blasts is one of the growth-inhibitory mechanisms of quercetin in these cells. This observation was supported by the following data. (1) Quercetin-sensitive leukemic blasts, when treated with quercetin, secrete large amounts of TGF-beta 1 in the medium and show positivity for TGF-beta 1-immunoreactive material in the cytoplasm. (2) At a concentration of 8 mumol/L, antisense TGF-beta 1 oligonucleotides prevent the growth-inhibitory action of quercetin. (3) Anti-TGF-beta 1 neutralizing monoclonal antibodies can prevent almost completely the growth-inhibitory activity of quercetin. The analysis of quercetin-resistant cases confirmed as well the central role of TGF-beta 1 in the growth-inhibitory activity of quercetin. In conclusion, quercetin can act as a cytostatic agent for leukemic cells by modulating the production of TGF-beta 1.