Circulating immune complexes (CIC), isolated from the serum of a patient with Hodgkin's disease (HD) and from control serum (CS) of healthy adults, were used to generate heterologous antisera in rabbits. The antiserum directed against CIC from HD (AS-HD) and the antiserum directed against CIC from CS (AS-CS) were used to identify immunoglobulins, complement factors and alpha2-macroglobulin as immune complex components. After adsorbing both antisera with normal human sera, we found that the adsorbed AS-HD was immunoreactive with radio-labelled CIC from HD serum but not with radiolabelled CIC from CS. Sera of patients with different diseases and sera of healthy adults were assessed for the occurrence of this Hodgkin immune complex-associated antigen (HIC-Ag). The HIC-Ag was present in 37% (12/33) of sera from patients with HD, 8% (8/101) of sera from patients with nonmalignant diseases, and 0% (0/6) of sera from healthy adults. This antigen was equally distributed among HD patients with and without symptoms, but its occurrence correlated with an advanced clinical stage of the disease. Using the adsorbed AS-HD in the immunoperoxidase technique, we identified the HIC-Ag as a cytoplasmic antigen in Hodgkin and Reed-Sternberg cells; whereas, the adsorbed AS-CS did not reveal any staining. These data indicate the presence of an HIC-Ag in the sera of patients with HD and suggest that the adsorbed AS-HD might be useful for isolation and characterization of this antigen for future use as a tumour marker.