This study investigates the expression of mRNA for Interleukin-5 (IL-5) in bronchial mucosal biopsies from asthmatic patients and non-atopic controls using in situ hybridization. The researchers obtained bronchial biopsies from 10 asthmatic subjects and 9 non-atopic controls. A radio-labeled cRNA probe derived from IL-5 cDNA was used, and the hybridization signals were detected under autoradiography. Six out of 10 asthmatic subjects showed specific hybridization signals for IL-5 mRNA within the bronchial mucosa, located beneath the epithelial basement membrane. No hybridization was observed in the control group. The six IL-5 mRNA-positive asthmatics tended to have more severe disease, as assessed by symptoms and lung function, and exhibited a significant increase in the infiltration of the bronchial mucosa by eosinophils and activated T lymphocytes. The study provides evidence for the cellular localization of IL-5 mRNA in the bronchial mucosa of asthmatics and supports the role of IL-5 in regulating eosinophil function in bronchial asthma.This study investigates the expression of mRNA for Interleukin-5 (IL-5) in bronchial mucosal biopsies from asthmatic patients and non-atopic controls using in situ hybridization. The researchers obtained bronchial biopsies from 10 asthmatic subjects and 9 non-atopic controls. A radio-labeled cRNA probe derived from IL-5 cDNA was used, and the hybridization signals were detected under autoradiography. Six out of 10 asthmatic subjects showed specific hybridization signals for IL-5 mRNA within the bronchial mucosa, located beneath the epithelial basement membrane. No hybridization was observed in the control group. The six IL-5 mRNA-positive asthmatics tended to have more severe disease, as assessed by symptoms and lung function, and exhibited a significant increase in the infiltration of the bronchial mucosa by eosinophils and activated T lymphocytes. The study provides evidence for the cellular localization of IL-5 mRNA in the bronchial mucosa of asthmatics and supports the role of IL-5 in regulating eosinophil function in bronchial asthma.