Excessive mucus causes severe airflow obstruction in fatal asthma. It is also present in mild to moderate disease, but is poorly understood and treated. Mucus overproduction is associated with dysregulated expression of the mucins MUC5AC and MUC5B. Whereas increased MUC5AC is a consistent finding, MUC5B varies—remaining stably produced in some patients but strongly repressed in others (>90%). Patients with lower MUC5B display worsened asthma phenotypes including airway hyperreactivity (AHR) to methacholine (MCh) and eosinophilic inflammation. To better understand the roles of mucins in asthma, we generated Muc5ac and Muc5b knockout (^−/−) mice. AHR to MCh was abolished in antigen-challenged Muc5ac^−/− mice, due to prevention of heterogeneous mucous plugging that occurred in allergic wild-type mice during MCh-induced bronchoconstriction. Thus, in addition to the established role of smooth muscle–mediated airway narrowing, Muc5ac is an essential noncontractile AHR component. We also found that, unlike Muc5ac^−/− mice, Muc5b-deficient mice were not protected from asthma phenotypes. Furthermore, whereas inflammation was unaffected by Muc5ac deficiency, it was exaggerated in the absence of Muc5b. On the basis of these differential effects, we are now determining how asthma phenotypes are regulated by mucin isoform specificity. Glycosylation is dramatically different: Muc5ac is heavily fucosylated whereas Muc5b is mainly sialylated. Fucosylation increases mucus viscoelasticity, and FUT2, the enzyme that catalyzes mucin α1,2-fucosylation, is associated with severe asthma exacerbation risk. Sialylation is required for binding to siglec (sialic acid–binding immunoglobulin-like lectin) receptors on leukocytes. Eosinophils express Siglec-F (mouse) or Siglec-8 (human). Engagement by sialoside ligands induces eosinophil apoptosis, and Muc5b via sialylated termini that require the α2,3-sialyltransferase ST3Gal3 for synthesis binds Siglec-F and induces apoptosis in mouse eosinophils. Because Muc5b is required for host defense in mouse lungs, inhibiting MUC5AC while preserving or enhancing MUC5B functions may be effective for treating asthma.