SummaryCharacterization of the immunological response to Alternaria in sensitive subjects is not complete. We used crossed radioimmunoelectrophoresis (CRIE) to identify antigens in Alternaria extracts reacting with IgE antibody in five patients with Alternaria‐sensitive asthma, four with Alternaria‐induced rhinitis, three non‐allergic asthmatics, and three normal controls. All five Alternaria‐asthma patients and three of four Alternaria‐rhinitis patients showed IgE binding to a single antigen peak in the CRIE. One patient exhibited specific radio‐staining to a second peak, and two other patients had IgE binding to a third antigen. These results suggest an analogy of Alternaria antigens with that found in ragweed pollen extracts, i.e. that IgE antibody is directed against more than one antigen. Using the enzyme‐linked immunosorbent assay (ELISA), we found a significant difference (P<0·05, unpaired Student'st‐test) in igG binding between Alternaria‐sensitive asthmatics and normal controls. There was no apparent difference in IgG binding between untreated Alternaria‐sensitive asthmatics and those receiving hig