RBL cells provide a useful model of the IgE and antigen-dependent stimulus-secretion coupling of mast cells and basophils. We have measured cellular deformability to investigate the participation of cytoskeletal mechanical changes. Cross-linking cell-surface IgE-receptor complexes with multivalent ligands not only triggered secretion but also caused the cells to stiffen, i.e., to become more resistant to deformation. This mechanical response required receptor cross-linking, had a time course similar to that of secretion, and was reversed by DNP-L-lysine, a competitive inhibitor of antigen binding. Hence the same stimulus seems to elicit both stiffening and secretion. Cytochalasin D, which inhibits actin filament assembly, prevented or reversed stiffening, thereby implicating the cytoskeleton in the mechanical response. Increasing intracellular calcium ion concentration with the ionophore A23187 stiffened cells and stimulated secretion. Activation of protein kinase C with a phorbol ester also stiffened cells and enhanced both the stiffening and secretion caused by the ionophore. Yet cytochalasin D enhances secretion whereas activation of protein kinase c alone is insufficient for secretion. Therefore stiffening is neither necessary nor sufficient for secretion. These results characterize a cytoskeletal mechanical response triggered by the same receptor-dependent stimulus that elicits secretion and by second messengers that are thought to mediate between the receptor signal and secretion. The function of the mechanical response, however, remains to be determined.