The levels of blocking antibodies were studied in 200 patients affected with bronchial asthma and/or allergic extrinsic rhinitis with sensitization to grass pollen and dust mites shown by cutaneous and RAST tests. IgG blocking antibodies were studied using ELISA, in relation to: Immunotherapy treatment duration. The degree of clinical improvement obtained. The conditions, before and after immunotherapy treatment. The conditions, 12-24 months after having completed 3 years of immunotherapy. The results indicate that: A high level of blocking antibodies is reached in the group of patients between 4-7 months of immunotherapy treatment. These patients also reached the stage of the maintenance dosage. There exists a significant and specific response of blocking antibodies in patients treated with specific immunotherapy. There exists a close correlation between symptomatology and the level of blocking antibodies, although this correlation is not perfect. The hyposensitization preparations used did not succeed in maintaining permanently high levels of blocking antibodies once the treatment was completed. ELISA is a good alternative to other more frequently used techniques in measuring blocking antibodies, and it can be used in following patients during the course of immunotherapy.