The paper examines the allergen specific B-cell response in peripheral blood from patients undergoing immunotherapy with house dust mite extract. The 12 patients were part of a double blind placebo controlled study, and they were treated with either Dermatophagoides pteronyssinus (n= 4), Dermatophagoides farinae extract (n= 3) (Alutard SQ, ALK, Denmark) or placebo (n= 5). Blood was taken every fortnight on day seven after hyposensitization and tested for IgM, IgG, IgA and IgE antibody secreting cells (AbSC) to D. pteronyssinus and D. farinae allergens and for the total number of immunoglobulin secreting cells (IgSC). The data showed a maximum of approximately 120 Der f I + II specific AbSC/106 mononuclear cells (MNC). A comparison of specific AbSC to the major allergens of the two house dust mites demonstrated that there was no measurable species specificity in the B-cell response that could be correlated to immunotherapy with either of the two extracts. The specific IgM, IgG, and IgA response to Der f I + II was examined in the placebo (39 measurements) and the actively treated (56 measurements) groups, and the results demonstrated a significant rise in specific IgM and IgA AbSC following immunotherapy. The number of specific IgG AbSC did not change. There was a mean of less than one specific IgE AbSC/106 MNC, and no detectable change following the treatment. It is speculated that immunotherapy to inhalant allergens causes the induction of specific IgA AbSC. It would then be these partly differentiated plasma cells that are detected on their way to the bronchial or gut mucosa to exert their protective function mediated by allergen specific secretory IgA.