Factor XIa is a serine protease which participates in the instrinsic pathway of coagulation by catalysing the conversion of factor IX to factor IXa.

Factor XIa is prepared by activating purified factor XI with factor XIIa. Following activation, factor XIa is purified to homogeneity by a combination of affinity chromatography techniques. The factor XIa is supplied in 50% (vol/vol) glycerol/H₂O and should be stored at −20 ˚C. Purity is assessed by SDS PAGE analysis. Activity is determined in a clotting assay using factor XI deficient plasma.