Fc alpha receptors (Fc alpha R), detected by the binding of IgA and by anti-Fc alpha R antibodies, were found to be differentially expressed on eosinophils and neutrophils. Neutrophils were the major granulocyte population expressing Fc alpha R, and they expressed much higher levels of Fc alpha R than eosinophils. The expression of Fc alpha R 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 Fc alpha R expression, whereas neutrophils did not. The Fc alpha R 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 Fc alpha R of eosinophils and neutrophils revealed a major protein core of 32 kD for both cell types. The data indicate that expression of Fc alpha R molecules with a characteristic glycosylation pattern is upregulated on eosinophils in allergic individuals.