Factor XI or plasma thromboplastin antecedent is the zymogen form of factor XIa, one of the enzymes of the coagulation cascade. Like many other coagulation factors, it is a serine protease. In humans, Factor XI is encoded by the F11 gene.

Successful perioperative management of factor XI deficiency with administration of [[fresh frozen plasma]] in a [[subdural hematoma]] patient
((Edahiro Y, Ichikawa K, Suzuki H, Yasuda H, Koike M, Komatsu N. Successful
perioperative management of factor XI deficiency with administration of
fresh-frozen plasma in a subdural hematoma patient. Geriatr Gerontol Int. 2016
Jan;16(1):143-144. doi: 10.1111/ggi.12512. PubMed PMID: 26974602.)).