Background: Eosinophils are a source of cytokines within the airways of asthmatic individuals that may exert an important immunoregulatory influence. Objectives: We examined IL-12 messenger (m)RNA and protein expression in eosinophils from peripheral blood and bronchoalveolar lavage (BAL) fluid obtained from subjects with atopic asthma (n = 7), patients with chronic bronchitis (n = 5), and nonatopic healthy control subjects (n = 7). To further define this IL-12+ population of eosinophils for the expression of other cytokines, we colocalized IL-12 and IL-5 within the peripheral blood eosinophils. Methods: To detect IL-12 mRNA and protein expression, we used in situ hybridization and immunocytochemistry techniques. The double-immunocytochemistry technique was used to localize IL-12 protein to BAL eosinophils and to colocalize IL-5 and IL-12 in peripheral blood eosinophils. Results: IL-12 mRNA and immunoreactive protein were localized to peripheral blood eosinophils. BAL fluid–derived eosinophils from asthmatic subjects were also reactive to IL-12. The percentage of peripheral blood eosinophils expressing mRNA for IL-12 was significantly lower in asthmatic subjects compared with that found in eosinophils obtained from patients with chronic bronchitis (P < .001) and control patients (P < .05). Colocalization studies demonstrated that the percentages of IL-12+ eosinophils that are also IL-5+ were 72% in asthmatic subjects and only 11% in control subjects (P < .001). Conclusion: These results suggest that eosinophils are a potential source of IL-12. Eosinophil-derived IL-12 may contribute and modulate the local allergic inflammatory responses. (J Allergy Clin Immunol 2000;106:288-92.)