An acquired IgG inhibitor to factor XIII was identified in a 30-year-old Vietnamese woman with systemic lupus erythematosus. The IgG fraction was isolated from plasma by chromatography on an agarose gel column and by protein G adsorption. The anti-factor XIII activity, identified within the IgG fraction, inhibited factor XIII from both normal plasma (a2b2) and platelets (a2). A normal level of the b subunit was measured by immunoelectrophoresis of the patient's plasma, but the a subunit was not detected. These results suggested the presence of an IgG immunoglobulin which recognized the a subunit and interfered with its activity.