Sera from forty-nine patients with atopic allergy were tested for the presence of circulating immune complexes by the sensitive, quantitative Clq assay. Compared to twenty-five normal individuals the atopic patients had significantly higher mean values of Clq reactive material, expressed as Clq inhibition values. The highest mean values were seen in the group (twenty-one patients) who had been on long term maintenance immunotherapy for 5 or more years. When positive sera were fractionated by ultracentrifugation on sucrose density gradients, the Clq reactive material was found in intermediate (7S–19S) and heavy (>19S) regions. The heavy material contained both IgM and IgG. It is suggested that the IgG antibodies known to be produced by chronic hyposensitization procedures in allergic individuals may circulate in the serum as antibody–allergen immune complexes, and could carry out their blocking action in this form.