Fcα receptors (FcαR), detected by the binding of IgA and by anti-FcαR antibodies, were found to be differentially expressed on eosinophils and neutrophils. Neutrophils were the major granulocyte population expressing FcαR, and they expressed much higher levels of FcαR than eosinophils. The expression of FcαR by eosinophils could be upregulated approximately threefold by Ca2+ ionophore treatment in a dose- and time-dependent manner. This effect, which was blocked by a chelating agent, was not duplicated by other cellular stimuli. Eosinophils in allergic individuals displayed enhanced FcαR expression, whereas neutrophils did not. The FcαR on eosinophils had a higher molecular mass (70-100 kD) than those identified on neutrophils (55-75 kD). However, removal of N-linked carbohydrates from FcαR of eosinophils and neutrophils revealed a major protein core of 32 kD for both cell types. The data indicate that expression of FcαR molecules with a characteristic glycosylation pattern is upregulated on eosinophils in allergic individuals.