The presence of cryoglobulin in plasma was found to interfere with the assessment of euglobulin fibrinolytic activity. Cryoglobulins co-precipitate with the isoelectrically precipitated euglobulins thereby giving rise to erroneous determinations of components of the extrinsic and intrinsic fibrinolytic systems. Cold-promoted activation of the F XII-dependent part of the intrinsic proactivator system was related to the presence of cryoglobulins. Strictly optimized procedures for the preparation of euglobulin solutions are essential in the accurate determination of euglobulin fibrinolytic activity. Resuspended euglobulins should be kept for at least 30 min at 0 degree C followed by at least 2 min at 37 degrees C in order to secure conditions yielding accurate and reproducible assays of activity in samples of patient plasma.