With the present study, evidence is provided that prekallikrein (PK) in human plasma might be present in two different states, one of them removed along with IgG on Protein G columns. At a plasma dilution of 1 + 2.5, small amounts of an IgG fraction were left in plasma along with all of the PK. At a dilution of 1 + 11, nearly all IgG was removed. The removal in parallel of part of the PK was shown in immunoblot experiments and confirmed in amidase assays. One monoclonal antibody against PK (13G11) and two preparations of polyclonal antibodies were used for the immunoblot experiments. Different peptide substrates (S-2302, S-2222, Bz-Pro-Phe-Arg-pNA), along with protease inhibitors (soybean trypsin inhibitor, corn trypsin inhibitor, lima bean trypsin inhibitor) were used for the amidase assays. The amidase assays indicated that factors XII and XI were reduced by Protein G columns. In all experiments with extensive removal of IgG, protein recognized by the factor XII light chain mAb C6B7 was removed at the same time. This antibody preparation did not detect purified contact factors, but it did recognize a preparation of purified beta-FXIIa, and also significant amounts of protein present in plasma deficient in factor XII and not detectable in plasma deficient in PK. This protein accordingly seems to be connected with the PK fraction removed with IgG.