Pathologic fibroblast proliferation or tissue fibrosis develops in certain chronic allergic diseases and in a wide array of other inflammatory disorders in which mast cell activation is also a prominent feature. In this study we investigated a number of potential mechanisms by which IgE-dependent activation of mouse mast cells might influence the proliferation of mouse fibroblasts in vitro. We found that supernatants from in vitro-derived mast cells that had been activated by IgE and specific antigen (but not those from quiescent mast cells) promoted the proliferation of mouse embryonic skin or 3T3 fibroblasts, and we showed that this effect was detectable in the absence of fetal calf serum. We analyzed the kinetics with which the fibroblast-proliferative activity was secreted from bone marrow–derived cultured mast cells and found that it was released both rapidly (i.e., in 30 minutes or less) and for a more prolonged period (i.e., for more than 2 hours) after IgE-dependent mast cell activation. We then measured the levels at which the mast cells produce a number of cytokines that are known to affect fibroblasts (IL-1, IL-6, transforming growth factor-β1 [TGF-β1], and tumor necrosis factor-⍺ [TNF-⍺]) and assessed their relative effects, as recombinant cytokines, on fibroblast proliferation. Our mast cells secreted high levels of TGF-β1 and TNF-⍺, intermediate amounts of IL-6, and low levels of IL-1. We titrated the fibroproliferative effects of each of these cytokines and determined that at a dose of 50 pg/ml their rank order of activity was TGF-β1 > TNF-⍺ > IL-1  > IL-6, with all but IL-6 having significant effects. The ability of supernatants from activated bone marrow–derived cultured mast cells to promote fibroblast proliferation was partially diminished by absorption with neutralizing antibodies against either TNF-⍺ or TGF-β1, and absorption of the supernatants with a combination of antibodies against TNF-⍺ and TGF-β1 reduced their ability to induce fibroblast proliferation by approximately 50% (p ≤ 0.001, n = 5). These findings show that IgE-dependent activation of mouse mast cells can result in the release of mediators that promote fibroblast proliferation in the absence of any other cell type and suggest that mast cell–derived TNF-⍺ and TGF-β1 contribute substantially to this effect. They also suggest that these cytokines exert their effects through synergistic interactions with other mast cell mediators. (J Allergy Clin Immunol 1997;99:113-23.)