Recently direct evidence for the role of interleukin-5 (IL-5) in eosinophilic inflammation in the airways of persons with asthma has been provided by an in situ hybridization study that used radioisotope-labeled IL-5 complementary RNA probes. Radioisotope-labeled probes, although sensitive, require autoradiographic detection, which is time-consuming. In the most recent study we attempted to detect IL-5 messenger RNA in the bronchial biopsy specimens from patients with asthma using nonradioactive in situ hybridization, which gives rapid results. Bronchial biopsy specimens were obtained from eight patients with asthma and seven diseased control subjects. IL-5 complementary DNA probes were labeled with digoxigenin-deoxyuridine triphosphate and hybridized to permeabilized sections. Hybridization signals were visualized by an immunohistochemistry technique. Positive hybridization signals were observed in six of the eight biopsy specimens from patients with asthma. Pretreatment with ribonuclease or hybridization with an unrelated probe produced negative results. Immunohistochemical staining of serial sections with a monoclonal antibody to IL-5 revealed that a few cells within the mucosa positively stained, suggesting active synthesis of IL-5. Biopsy results from the seven diseased control subjects did not show any hybridization signal. These results confirm and extend previous observations of IL-5 messenger RNA expression in the airways of patients with asthma, and suggest that digoxigenin-labeled IL-5 complementary DNA probes would be a powerful research tool.