The allergic reaction begins with the antigen-induced aggregation of occupied high-affinity IgE receptors expressed on mast cell surface, their activation, and the release of proinflammatory mediators that cause the “early phase” of this process. In addition, mast cell activation induces the onset of a “late phase” reaction characterized by the tissue infiltration of inflammatory cells, mainly eosinophils. We have hypothesized that during the late phase mast cells interact with and are activated by eosinophils. Here we report that highly purified human lung mast cells became responsive to eosinophil major basic protein (MBP) when in coculture with human lung fibroblasts. In addition, cord blood–derived mast cells maintained in coculture with 3T3 fibroblasts released more histamine and prostaglandin D₂ (PGD₂) compared with cells maintained in suspension. The fibroblast-derived membrane form of stem cell factor (SCF) was found to be involved in the mast cell increased responsiveness to MBP. In fact, cord blood–derived mast cells cocultured with 3T3 in the presence of antisense for SCF or cocultured with fibroblasts that do not express the membrane form of SCF were inhibited in their histamine-releasing activity toward MBP. In addition, this form of SCF induced the expression of a pertussis toxin–sensitive G_i protein, G_i3 that interacts with MBP to trigger mast cell non-IgE–dependent activation in a manner similar to other cationic compounds such as compound 48/80. Mast cell responsiveness to eosinophil mediators is a potentially novel evidence for an alternative pathway of allergen-independent activation able to contribute to the perpetuation of allergy.