Abstract. In this study, we examined the role of the bumetanide-sensitive Na+/K+/CI -cotransport in the mitogenic signal of human skin fibroblast proliferation . The Na+/K+/Cl -cotransport was dramatically stimulated by either fetal calf serum, or by recombinant growth factors, added to quiescent Go/Gt human skin fibroblasts . The following mitogens, FGF, PDGF, a-thrombin, insulin-like growth factor-1, transforming growth factor-a, and the phorbol ester, 12-0-tetradecanoyl-phorbol-13-acetate, all stimulated the Na+/K+/Cl -cotransport . In addition, all the above mitogens induced DNA synthesis in the synchronized human fibroblasts. In order to explore the role of the Na+/K+/Cl -cotransport in the mitogenic signal, the effect of two specific inhibitors of the cotransport, furosemide and bumetanide, was tested on cell proliferation induced by the above recombinant growth NE of the earliest responses of quiescent cells to a mitogenic signal is activation of Na+ influx (Rozengurt, 1986;Rozengurt and Mendoza, 1986) . The bumetanide-sensitive Na+/K+/Cl -cotransport was shown to be dramatically stimulated by the addition of serum growth factors to quiescent cells (Tupper et al., 1977; Pallet et al., 1982 Pallet et al., , 1983Panet, 1985;Amsler et al., 1985;Paris and Pouyssegur, 1986; Panet et al., 1986aPanet et al., , 1989. The question of whether the Na+/K+/Cl-cotransport has an essential role in the mitogenic response is still unsolved . Several groups have observed that inhibition of the Na+/K+/Cl-cotransport affected only slightly the initiation of DNA synthesis (Owen and Prastein, 1985 ;Amsler et al., 1985;Paris and Pouyssegur, 1986) . It has therefore been proposed that the Na+/K+/Cl-cotransport does not play a major role in the mitogenic signal (Amsler et al ., 1985 ;Paris and Pouyssegur, 1986). Most ofthese studies, however, were carried out with immortal rodent cell lines. Nevertheless, the response of normal diploid human fibroblasts, rather than immortal cell lines to growth factors, may be different and more relevant to the normal control of cell proliferation . Recently, we have investigated the optimal conditions for thearrest ofhuman skin fibroblasts . Optimal cell density, du-0 The Rockefeller University Press, 0021-9525/91/07/337/6 $2 .00 The Journal of Cell Biology, Volume 114, Number 2, July 1991 337-342 factors. Bumetanide and furosemide inhibited synchronized cell proliferation as was measured by (a) cell exit from the Go/G, phase measured by the use of flow cytometry, (b) cell entering the S-phase, determined by DNA synthesis, and (c) cell growth, measured by counting the cells. The inhibition by furosemide and bumetanide was reversible, removal of these compounds, completely released the cells from the block of DNA synthesis . In addition, the two drugs inhibited DNA synthesis only when added within the first 2-6 h of cell release. These results indicate that the effect of these drugs is specific, and is not due to an indirect toxic effect . This study clearly demonstrates that the growth facto...