Because both hyperosmolar and IgE-dependent stimuli may be encountered in the airway, and because hyperosmolarity causes histamine release in basophils, we examined the effects of the 2 stimuli on human lung mast cells. Mast cells prepared by enzymatic digestion of human lung were suspended in buffers made hyperosmolar with mannitol. Significant histamine release was seen above 360 mOsm/kg H2O, increasing to 11.9 +/- 1.0% at 770 mOsm/kg H2O, and release was synergistically enhanced by anti-IgE. Cells that had been rendered unresponsive to IgE-dependent stimuli by exposure to anti-IgE in the absence of Ca++ became markedly more responsive to hyperosmolar stimulation, and released as much as 32 +/- 2% histamine in hyperosmolar buffers alone. Antigen-induced histamine release from the basophils of allergic donors was also synergistically enhanced in buffers above 460 mOsm/kg H2O. These data show that immunologic and nonimmunologic stimuli may interact, and that human lung mast cells are capable of mediator release when exposed to osmolarities that may occur in the airway, especially during hyperventilation. Hyperosmolar mediator release is a plausible mechanism by which exercise-induced hyperventilation might induce asthma.