IgE mediates allergic reactions by binding to the high-affinity receptor, Fc epsilonR1, on mast cells and basophils at mucosal surfaces; then cross-linking of the receptor by multivalent antigen triggers the allergic response. We demonstrate here that B cells in the nasal mucosa of patients with hay fever express IgE. The results also suggest that allergen-induced heavy-chain switching to IgE occurs locally within the nasal mucosa. Local IgE synthesis may explain why some 'atopic' patients develop rhinitis whereas others have either no clinical manifestations or develop atopic disease elsewhere.