Sera from forty patients with house-dust-mite-sensitive bronchial asthma were examined for the presence of circulating immune complexes (CIC) by the sensitive and quantitative Clq solid-phase radioimmunoassay (Clq-SP) and a monoclonal rheumatoid factor solid-phase radioimmunoassay (mRF-SP). Compared to fifteen normal individuals, the asthmatic patients showed significantly higher mean values of Clq-reactive materials; however, there was no difference between the results from the patients treated by immunotherapy using Dermatophagoides farinae extract and those not so treated. Moreover, immune complexes in eight patients before and after immunotherapy showed that the amount of the complexes tend to decrease during immunotherapy. Furthermore, the presence of complexes had no relationship with the amounts of mite-specific IgG antibody. Similar results were also obtained in tests using mRF-SP. These data suggest that complexes in the sera of the house-dust-mite-sensitive asthmatic patients are not necessarily associated either with immunotherapy or with the mite-specific IgG antibodies.