Pulmonary fibrosis is a potentially fatal consequence of treatments for malignancy and is an increasing problem in bone marrow transplant patients and in cases of allogeneic lung transplant. The fibrotic response is characterized by increases in lung fibroblast number and collagen synthesis. This laboratory previously isolated stable, functionally distinct, murine lung fibroblast subsets (Thy‐1⁺ and Thy‐1⁻) to study the contribution of fibroblast subpopulations in lung fibrosis. The fibroblast fibrotic response may be induced by cytokines secreted by infiltrating cells such as T lymphocytes and mast cells. In the current study two key regulatory cytokines, interferon‐γ (IFN‐γ) and interleukin‐4 (IL‐4), were investigated for their effects on the collagen synthesis of murine lung fibroblast subsets. IL‐4 and IFN‐γ are putatively characterized as fibrogenic and anti‐fibrogenic cytokines, respectively, and are found in repairing lung tissue. Stimulation with recombinant IL‐4 induced a 100% increase in total collagen production only by Thy‐1⁺ fibroblasts. Types I and III collagen mRNA were increased in the Thy‐1⁺ fibroblasts, unlike the Thy‐1⁻ subset. In contrast, IFN‐γ decreased constitutive collagen production by more than 50% in Thy‐1⁺ and Thy‐1⁻ fibroblasts. Interestingly, the two subsets utilized their collagen production machinery (collagenase, tissue inhibitors of metalloproteinases) differently to further regulate collagen turnover in response to IL‐4 and IFN‐γ. Overall, our data support the hypothesis that IL‐4 is fibrogenic and IFN‐γ is anti‐fibrogenic. Moreover, selective expansion of IL‐4 responsive fibroblasts (e.g., Thy‐1⁺) may be important in the transition from repair to chronic fibrosis. In addition, these data suggest that an inflammatory response dominated by IL‐4‐producing Th₂ lymphocytes and/or mast cells will promote fibrosis development. © 1996 Wiley‐Liss, Inc.