Factor XI (FXI) deficiency is associated with an abnormal bleeding state. The extent of bleeding does not correlate well with the plasma concentration of FXI, and it has been suggested that also unknown factors interfere with the bleeding tendency. In a recent paper (Thromb. Res. 74, 477-485, 1994) we found that FXIa activated in human plasma was present in association with part of factor XIIa (FXIIa) and part of kallikrein, influencing their functional activities. Should the activity level of FXIa also be altered by the other contact factors this might provide one approach to the problem of the failure of assays of FXIa to correlate with bleeding tendency. In the present study we have developed an assay procedure for FXIa based on its amidolytic (S-2366) activity, and allowing at the same time a quantification of the amount of FXIa associated to kallikrein. The total amidase activity obtained was separated into two main fractions by use of soybean trypsin inhibitor (STI), corn inhibitor (CI) and lima bean trypsin inhibitor (LTI). One fraction contained free FXIa which could be specifically blocked by LTI. An inhibitor resistant fraction was found to contain FXIa inactive in association with kallikrein. The content of FXIa could be assessed in experiments with mixtures of normal plasma and plasma deficient in prekallikrein, and was taken into account in the calculations. This fraction increased during storage of plasma at -70 degrees C. To obtain stable and comparable assay conditions the method was based on plasma stored for at least four weeks. The specificity of the method was verified by parallel radial immunodiffusion tests. The results imply that the activity level of FXIa is dependent on kallikrein present. If the experimental results has relevance to the situation under physiological conditions, they indicate one possible cause of the failure of assays of FXI to correlate with bleeding tendency.