Mediators released from human basophils and mast cells in response to immunologic and other stimuli are felt to be important in the pathophysiology of several nasal and pulmonary disease processes. Recently, we have developed techniques to purify these cells, thus allowing precise ultrastructural, biochemical and pharmacological studies of mediator release. Previous literature has emphasized the similarities of the two cell types including their metachromatic staining and IgE-mediated release of mediators. However, we now appreciate that several differences exist between the two cell types. At the ultrastructural level, basophil release is characterized by individual granules emptying to the cell exterior, while mast cell granules fuse intracellularly and release their contents through cytoplasmic channels. Functionally, basophils are 10- to 30-fold more sensitive than mast cells to anti-IgE, but the kinetics of release are less rapid. Basophil, but not mast cell, release is (I) inhibited by histamine H2 agonists and glucocorticoids; (II) enhanced by PgD2 and (III) modulated by arachidonic acid lipoxygenase metabolites. Significant quantities of slow-reacting substance of anaphylaxis, PgD2 and platelet-activating factor are generated by mast cells. Basophils produce little or none of these mediators. Studies with these purified, relevant human cell types should provide important insights into the cellular basis of hypersensitivity states and their control.