Uncontrolled fibroblast activation is one of the hallmarks of fibrotic lung disease. Prostaglandin E₂ (PGE₂) has been shown to inhibit fibroblast migration, proliferation, collagen deposition, and myofibroblast differentiation in the lung. Understanding the mechanisms for these effects may provide insight into the pathogenesis of fibrotic lung disease. Previous work has focused on commercially available fibroblast cell lines derived from tissue whose precise origin and histopathology are often unknown. Here, we sought to define the mechanism of PGE₂ inhibition in patient-derived fibroblasts from peripheral lung verified to be histologically normal. Fibroblasts were grown from explants of resected lung, and proliferation and collagen I expression was determined following treatment with PGE₂ or modulators of its receptors and downstream signaling components. PGE₂ inhibited fibroblast proliferation by 33% and collagen I expression by 62%. PGE₂ resulted in a 15-fold increase in intracellular cAMP; other cAMP-elevating agents inhibited collagen I in a manner similar to PGE₂. These effects were reproduced by butaprost, a PGE₂ analog selective for the cAMP-coupled E prostanoid (EP) 2 receptor, but not by selective EP3 or EP4 agonists. Fibroblasts expressed both major cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP-1 (Epac-1), but only a selective PKA agonist was able to appreciably inhibit collagen I expression. Treatment with okadaic acid, a phosphatase inhibitor, potentiated the effects of PGE₂. Our data indicate that PGE₂ inhibits fibroblast activation in primary lung fibroblasts via binding of EP2 receptor and production of cAMP; inhibition of collagen I proceeds via activation of PKA.