Mg2+ anticoagulated PPP (20mM MgCl2) was applied at RT to a Zn2+ immobilised biscarboxymethylamino Sepharose 4B column and eluted with 20mM Tris, 20mM MgCl2, 2.5mM CaCl2 0.15M NaCl buffer pH 7.4. Following collection of the wash-through fractions, bound proteins were developed through application of linear (0 to 35mM) imidazole gradients.All fractions were screened for F.II, V, IX, X, vWF:Ag, Protein C, Fg, Fn and 2-macroglobulin by ELISA, VIII:Ag by IRMA, and VIII:C by both 1-stage and 2-stage bioassay methods. VIIIrAg (60-90% yield), vWF:Ag (100%), VIII:C 1-stage activity (35%), Fn (> 60%), Fg (> 80%), V (50%) and a2-macroglobulin (> 70%) were located only in the gradient fractions. Two distinct peaks demonstrated shortening of the 2-stage VIII:C assay:- one co-eluting with the 1-stage VIII:C activity and another major peak in the wash-through fractions where antigenic determinants of F.II, IX, X and part of the protein C were located and partially resolved from each other. Only F.II.-Ag co-eluted with the “ 2-stage VIII:C” activity. Similar observations were found in Mg2+ -anticoagulated severe Haemophilia A plasma and in citrated PPP developed with 20mM MgCl2, 20mM Tris buffer. A1(0H)3 treatment abolished the activity from citrate -, but not from Mg2+ -anticoagulated plasma.The relevant fractions showed no activities in F.V, VII, VIII:C, IX or X 1-stage bioassays. They did not clot Fg and did not contain detectable Xa or Ila as assessed by S-2222, S-2238 or S-2288. Following incubation with specific antisera against IgG, II, V, VII:Ag, X, protein C, a- and g- lipoproteins, and plasminogen only anti-II inhibited this "2-stage VIII:C" activity. 2-stage VIII:C assays depend upon Ca2+ -dependent generation of Xa. Since the activity in the wash-through fractions could not be ascribed to VIII, Xa, or Ila the results would indicate the presence of a hitherto undescribed Factor X activator activity.