Background: Myofibroblasts are one of the primary cell types responsible for the accumulation of extracellular matrix in fibrosing diseases, and targeting myofibroblast differentiation is an important therapeutic strategy for the treatment of pulmonary fibrosis. TGF-β has been shown to be an important inducer of myofibroblast differentiation. We previously demonstrated that lactate dehydrogenase and its metabolic product lactic acid are important mediators of myofibroblast differentiation, via acid-induced activation of latent TGF-β. Here we explore whether pharmacologic inhibition of LDH activity can prevent TGF-β induced myofibroblast differentiation. Methods: Primary human lung fibroblasts from healthy patients and those with pulmonary fibrosis were treated with TGF-βand or Gossypol, an LDH inhibitor. Protein and RNA were analyzed for markers of myofibroblast differentiation and extracellular matrix generation. Results: Gossypol inhibited TGF-β induced expression of the myofibroblast marker α-smooth muscle actin (SMA) in a dose dependent manner in both healthy and fibrotic human lung fibroblasts. Gossypol also inhibited expression of collagen 1, collagen 3 and fibronectin. Gossypol inhibited LDH activity, the generation of extracellular lactic acid, and the rate of extracellular acidification in a dose dependent manner. Furthermore, Gossypol inhibited TGF-βbioactivity in a dose dependent manner. Concurrent treatment with an LDH siRNA increased the ability of Gossypol to inhibit TGF-β induced myofibroblast differentiation. Conclusions: Gossypol inhibits TGF-β induced myofibroblast differentiation through inhibition of LDH, inhibition of extracellular accumulation of lactic acid and inhibition of TGF-β bioactivity. These data support the hypothesis that pharmacologic inhibition of LDH may play an important role in the treatment of pulmonary fibrosis.