Clotting activities as well as thrombogenicity in rabbits of two activated prothrombin complex concentrates, Feiba (Immuno) and Autoplex (Travenol) were compared. When total activities of clotting factors were measured, factor VII was considerably higher in Autoplex than in Feiba. Likewise, a higher concentration of the activated serine proteases VIIa and IXa was found in Autoplex. On incubation with calcium chloride low activities of Xa and IIa appeared in both concentrates. When factor VIII was also added, the increase of these activities was higher in Autoplex but not in Feiba. On addition of phospholipid, the activities of Xa and IIa considerably increased in Autoplex but not in Feiba. There was a rapid diminution of these activities when antithrombin III was in the test system. Factor IXa however, on incubation of the concentrates with antithrombin III disappeared slower from Autoplex than from Feiba. Thrombogenicity in rabbits assayed by the Wessler test varied between different batches of the concentrate but was higher in Autoplex than in Feiba by an average factor of between 12.8 and 17.8.