Immunoconglutinins (Ic) are a group of predominantly IgM antibodies formed towards antigenic determinants exposed in fixed complement components (C3b and C4). Ic production is stimulated by bacterial and viral infections; elevated titers were also found in a number of other diseases involving completement-fixing reactions in vivo. High titers have been associated with the infectious morbidity within a particular population. The object of this investigation was to determine the correlations of autostimulated Ic titre with several parameters in 100 atopic nondesensitized subjects with respiratory allergies and in 140 non atopic, healthy, individuals. Eighty of them had rhinitis and/or conjunctivitis, 9 asthma and 11 rhinitis and asthma. All the atopics were studied also after at least 6 months of specific immunotherapy. None of the subjects received any other therapy at the time of study. Atopic subjects with high titers of Ic are significantly more numerous than controls, without significant correlation with the kind and beginning of atopic syndrome, and with immunotherapy. These results might be attributed to a higher incidence of infections in the atopic population. The Ic might play an important role in the immunoregulation involving complement system.