CD22 is currently recognized as a B cell-specific Siglec and has been exploited therapeutically with humanized anti-CD22 mAb having been used against B cell leukemia. In this study, tissue-specific eosinophil mRNA microarray analysis identified that CD22 transcript levels of murine gastrointestinal (GI) eosinophils are 10-fold higher than those of lung eosinophils. To confirm the mRNA data at the protein level, we developed a FACS-based protocol designed to phenotype live GI eosinophils isolated from the murine lamina propria. Indeed, we found that jejunum eosinophils expressed remarkably high levels of surface CD22, similar to levels found in B cells across multiple mouse strains. In contrast, CD22 was undetectable on eosinophils from the colon, blood, thymus, spleen, uterus, peritoneal cavity, and allergen-challenged lung. Eosinophils isolated from newborn mice did not express CD22 but subsequently upregulated CD22 expression to adult levels within the first 10 d after birth. The GI lamina propria from CD22 gene-targeted mice harbored more eosinophils than wild type control mice, whereas the GI eosinophil turnover rate was unaltered in the absence of CD22. Our findings identify a novel expression pattern and tissue eosinophilia-regulating function for the "B cell-specific" inhibitory molecule CD22 on GI eosinophils.