Mast cell - fibroblast interactions may contribute to fibrosis in asthma and other disease states. Fibroblast contraction is known to be stimulated by co-culture with the human mast cell line, HMC-1, or by mast cell-derived agents. Matrix metalloproteases (MMPs) can also mediate contraction, but the MMP-dependence of mast cell-induced fibroblast contractility is not established, and the consequences of mast cell activation within the co-culture system have not been fully explored. We demonstrate that activation of primary human mast cells (pHMC) with IgE receptor cross-linking, or activation of HMC-1 with C5a, enhanced contractility of human lung fibroblasts (HFL-1) in a three-dimensional collagen lattice system. This enhanced contractility was inhibited by the pan-MMP antagonist, batimastat, and was transferrable in the conditioned medium of activated mast cells. Exogenously added MMPs promoted gel contraction by mediating the proteolytic activation of latent TGF-. Consistent with this, fibroblast contraction induced by mast cell activation was enhanced by addition of excess latent TGF- to the cultures. Batimastat inhibited this response, suggesting that MMPs capable of activating latent TGF- were released following mast cell activation in co-culture with fibroblasts. Collagen production was also stimulated by activated mast cells, in an MMP-dependent manner. MMP-2 and MMP-3 content of the gels increased in the presence of activated mast cells, and inhibition of these enzymes blocked the contractile response. These findings demonstrate the MMP dependence of mast cell-induced fibroblast contraction and collagen production.