I. Introduction The basophilic leukocyte has long been known as a pivotal effector cell in anaphylaxis reactions (1). Although the basophil initially was regarded as the blood-borne counterpart of the mast cell, it is now clear that it is derived from a totally distinct lineage (2) and differs from the mast cell in many fundamental areas. Mast cells have been more extensively studied than basophils, since they are more readily identified and may be obtained in adequate quantities for investigation, and thus their involvement in asthma is well documented. Basophils, however, are less abundant, and there have previously been no known single specific markers for use in basophil identification; thus these cells have been the subject of relatively fewer studies. Despite this, there is accumulating evidence to support the active participation of basophils in the inflammation associated with asthma.