We characterized the existence, translocation, and reabsorption during cellular activation of a constitutively expressed intracellular CD16 in the human eosinophil. By two-color flow cytometry, we showed that 6.5+/-0.3% of nonpurified eosinophils expressed surface CD16. After digestion with phosphatidylinositol-specific phospholipase C, surface CD16 on both neutrophils and eosinophils decreased substantially, suggesting that eosinophil CD16 is a glycosyl-phosphatidylinositol-linked isoform. However, CD16 was substantially expressed intracellularly in human eosinophils. Epitope-specific binding to CLB-gran11 mAb from non-NA2/NA2 donors demonstrated that intracellular eosinophil CD16 also differed from the transmembrane isoform of CD16 expressed on NK cells or macrophages. Western blot analysis performed with 3G8 or DJ130c mAb showed a broad band at approximately 65 to 80 kDa, which was the same as neutrophil CD16 from the same NA2/NA2 donors. Upon stimulation by chemoattractants C5a, FMLP, or platelet-activating-factor, eosinophilic intracellular CD16 was rapidly translocated to the eosinophil surface, expressed maximally at 30 s, and then gradually disappeared from the cell surface during the next 10 min. Intracellular flow cytometry of stimulated eosinophils and sandwich ELISA of stimulated eosinophil supernatants demonstrated that the disappearance was due to its rapid release into medium and reabsorption by the cells. Our data identify a CD16B that is consistently expressed intracellularly but only rarely on the surface of nonactivated human eosinophils. This CD16 is transiently expressed during stimulation by chemoattractants.