During anaphylactic degranulation, basophilic leukocytes and mast cells stimulated by specific antigen or other appropriate agents rapidly undergo a series of ultrastructural alterations that expose the contents of their cytoplasmic granules to the external milieu. This noncytolytic process results in the release of several potent inflammatory mediators whose combined local and systemic effects largely determine the clinical manifestations of allergic disorders of immediate hypersensitivity. In this report we briefly review our ultrastructural studies of anaphylactic degranulation in basophils purified from the peripheral blood of guinea pigs and humans and in mast cells purified from human lung.