This study investigated the expression of IL-5 mRNA in bronchial mucosal biopsies from asthmatic patients and normal controls using in situ hybridization. Bronchial biopsies were obtained from 10 asthmatic patients and 9 nonatopic normal controls. A radio-labeled cRNA probe derived from IL-5 cDNA was used to detect IL-5 mRNA. The results showed that IL-5 mRNA was present in 6 out of 10 asthmatic subjects, with hybridization signals located beneath the epithelial basement membrane. In contrast, no hybridization was observed in the control group. The six IL-5 mRNA-positive asthmatics had more severe disease, as indicated by symptoms, lung function, and increased infiltration of eosinophils and activated T lymphocytes. There was a significant correlation between IL-5 mRNA expression and the number of CD25+ and EG2+ cells and total eosinophil count. The study provides evidence for the cellular localization of IL-5 mRNA in the bronchial mucosa of asthmatics and supports the concept that IL-5 regulates eosinophil function in bronchial asthma. IL-5 is a cytokine that promotes the differentiation of eosinophil precursors and enhances the effector capacity of mature eosinophils. It also prolongs the survival of eosinophils in vitro. The study used in situ hybridization with an IL-5 mRNA probe to investigate the expression of IL-5 mRNA and the distribution of IL-5-producing cells in bronchial tissue from asthmatic and normal individuals. The results suggest a possible link between eosinophils and T lymphocytes in asthma. The study also found that the expression of IL-5 mRNA was associated with the severity of the disease and the degree of infiltration of the airway mucosa with eosinophils and activated T lymphocytes. The findings support the hypothesis that IL-5 released by T lymphocytes is involved in eosinophil recruitment and activation in asthma.This study investigated the expression of IL-5 mRNA in bronchial mucosal biopsies from asthmatic patients and normal controls using in situ hybridization. Bronchial biopsies were obtained from 10 asthmatic patients and 9 nonatopic normal controls. A radio-labeled cRNA probe derived from IL-5 cDNA was used to detect IL-5 mRNA. The results showed that IL-5 mRNA was present in 6 out of 10 asthmatic subjects, with hybridization signals located beneath the epithelial basement membrane. In contrast, no hybridization was observed in the control group. The six IL-5 mRNA-positive asthmatics had more severe disease, as indicated by symptoms, lung function, and increased infiltration of eosinophils and activated T lymphocytes. There was a significant correlation between IL-5 mRNA expression and the number of CD25+ and EG2+ cells and total eosinophil count. The study provides evidence for the cellular localization of IL-5 mRNA in the bronchial mucosa of asthmatics and supports the concept that IL-5 regulates eosinophil function in bronchial asthma. IL-5 is a cytokine that promotes the differentiation of eosinophil precursors and enhances the effector capacity of mature eosinophils. It also prolongs the survival of eosinophils in vitro. The study used in situ hybridization with an IL-5 mRNA probe to investigate the expression of IL-5 mRNA and the distribution of IL-5-producing cells in bronchial tissue from asthmatic and normal individuals. The results suggest a possible link between eosinophils and T lymphocytes in asthma. The study also found that the expression of IL-5 mRNA was associated with the severity of the disease and the degree of infiltration of the airway mucosa with eosinophils and activated T lymphocytes. The findings support the hypothesis that IL-5 released by T lymphocytes is involved in eosinophil recruitment and activation in asthma.