Antibodies to factor XIII have not been previously characterized in detail. An acquired inhibitor to factor XIII, which developed in a patient following thrombectomy for a femoral artery occlusion, has been characterized as to immunoglobulin composition. A highly sensitive monodansylcadaverine assay for factor XIII was employed. The inhibitor was completely neutralized by antisera specific for the γG class, γG1 subclass, and λ light-chain type. Antisera to γG2, γG3, γG4, γA, γM, γD, γE, and k light-chains had no detectable effect. Preparative zone electrophoresis revealed the inhibitor to be in the cathodal portion of the gamma peak, with moderately restricted electrophoretic mobility. The factor XIII inhibitor is thus characterized as a γGl, λ immunoglobulin and resembles other human antibodies to clotting factors that are restricted in immunoglobulin composition.