Patients allergic to grass pollen were hyposensitized with two major allergenic components or whole extract of timothy grass pollen. Specific IgE, IgG1, and IgG4 formed during immunotherapy were analyzed by immunoblotting. Similar antibody-binding patterns were observed in both patient groups. Inhibition experiments using allergenic components isolated by preparative sodium dodecyl sulphate-polyacrylamide gel electrophoresis indicated that all antigenic components of timothy grass pollen detected in immunoblot dispose of private and cross-reactive determinants for binding of human IgE. The worse clinical outcome of immunotherapy after hyposensitization with two cross-reactive components did, therefore, not correlate with a specific antibody formation pattern.