House dust miteD. farinaeandDer f II‐specific IgG and IgG subclass responses were evaluated in 32 adults with perennial rhinitis undergoing immunotherapy for 1 year by means of IgG‐RAST and ELISA. The ELISA method, which is based on subclass‐specific monoclonal antibodies, could detect 0.5–1.5 ng/ml of specific antibodies.D. farinaeandDer f II‐specificIgG4antibodies increased continuously as immunotherapy proceeded, while total IgG and IgG1antibodies reached a plateau value 6 months after the start of immunotherapy, followed by a slow decrease during maintenance therapy. During the early phase of treatment the concentration of IgG, and IgG4antibodies rose, even though the increase of IgG4antibodies dominated. The positive clinical outcome as measured by the decrease in conjunctival sensitivity was associated with an increased ratio of specific IgG4/IgG, antibodies as well as the magnitude of the IgG4subclass response. Quantitation of IgG subclass antibodies in patients undergoing immunotherapy may be of some clinical value, but the clinical usefulness needs to be demonstrated for each type of allergen and possibly also for each ass