Human fibrin-stabilizing factor (Factor XIII) has been studied immunologically by the preparation of specific anti-Factor XIII antiserum in rabbits. On immunodiffusion it was found that normal plasma produced two precipitin lines. One of the precipitin lines was identical with that present in soluble platelet extract (the alpha-component), the other with that present in normal serum (beta-component). Plasma and serum of patients with congenital Factor XIII deficiency contained only the beta-component. By adsorption it was possible to prepare a second antiserum with solely anti-alpha-activity that did not react with the serum or plasma of XIII-deficient patients. Both antisera neutralized the clot-stabilizing activity of normal plasma. The action of thrombin on fibrinogen-free plasma or platelet extract abolished the immunoprecipitin alpha-line but did not reduce the capacity to neutralize antibody as measured by clot stabilization. It is concluded that the plasma Factor XIII molecule consists of two immunologically identifiable components, alpha and beta. The clot-stabilizing activity and thrombin-reactive site are situated on the alpha-component. Patients with congenital Factor XIII deficiency are devoid of immunologically identifiable or functional alpha-component but retain immunologically identifiable beta-component. It is this beta-component that accounts for the observed immunologically detectable Factor XIII in those patients devoid of clot-stabilizing activity.