Fibrinogen degradation products (FDP) interfere with coagulation of fibrinogen and may lead to incorrect estimates of fibrinogen in plasma and of FDP in serum. At pH 7.8 as much as 50–60 % of early FDP were incorporated into clots made from whole blood or from plasma, whereas late FDP were incorporated to minor extent. By lowering pH to 6.3, incorporation of early FDP was markedly reduced. The amount of unclotted fibrin due to the presence of FDP was also reduced. Thus, for a more correct estimate of FDP in serum, it is suggested that pH of blood or plasma should be lowered prior to clotting. Estimation of fibrinogen according to the Jacobsson assay (pH 6.3) is fairly exact in the presence of FDP, except at very high FDP concentrations.