Many antibody preparations exhibit proteolytic activity due to the presence of plasmin. In crossed immunoelectrophoresis at pH 8.6 this enzyme can degrade certain proteins during electrophoresis in the antibody-containing gel, resulting in artefacts in the form of extra precipitation arcs of congruent shape. The degradation behaviour of spectrin, a major protein of human erythrocyte membranes, was investigated. The artefact could be completely abolished by the addition of protease inhibitors, e.g. aprotinin and soya bean trypsin inhibitor, to the antibody preparations.