This article reports the rare occurrence and investigation of a specific anti-factor XI inhibitor that arose after fresh-frozen plasma infusion into a patient with previously unrecognized Factor XI deficiency. The IgG fraction of the patient's plasma that contained anti-Factor XI antibody was isolated by chromatography on DEAE-Affigel and concentrated. It was shown to exert inhibitory effect on purified Factor XI and XIa both in the activated and nonactivated partial thromboplastin time assay.