['Fibrinogen present in serum specimens can interfere with the interpretation of serum protein electrophoresis. We report here the unexpected precipitation of fibrinogen by an IgA antiserum used in immunofixation electrophoresis.', 'Immunofixation electrophoresis of plasma combined with ethanol precipitation, serial dilution of plasma, and fibrinogen adsorption of the antiserum were used to investigate the apparent immunoreactivity of a commercial source of IgA antiserum to fibrinogen.', 'Fibrinogen immunoreactivity by IgA antiserum was observed at fibrinogen concentrations > or =0.93 g/l. Ethanol precipitation of plasma removed fibrinogen and prevented the immunofixation of the protein by the IgA antiserum. Adsorption of the IgA antiserum with human fibrinogen removed its ability to precipitate fibrinogen, demonstrating cross-reactivity between the IgA antiserum and fibrinogen.'] Does immunofixation reveal an apparent alpha heavy chain caused by precipitation of fibrinogen with IgA antiserum?