A dysfibrinogenemia (fibrinogen Sevilla) was detected in a 64-yr-old woman with no previous history of hemorrhagic diathesis or thrombosis. Thrombin and reptilase times were prolonged. The aggregation of fibrin monomers showed a prolonged latency time with a defective slope although fibrinopeptide release and clot stabilization were found to be normal. Plasmin proteolysis was abnormal with a much slower plasmic degradation in patient's purified fibrinogen. By chromatofocussing the patient's fibrinogen showed an abnormality in pattern elution with a second peak eluting at a pH slightly more basic than the normal one (pH 5.5). Likewise, the isoelectrofocussing of purified non-reduced patient's fibrinogen in agarose gel showed an abnormal distribution in its focussed bands, especially in a group which focussed in a pI-interval between 5.20-5.85. By two-dimensional electrophoresis we did not find any abnormality in the fibrinogen-reduced chains. These results could indicate that the abnormal monomer aggregation, as well as the defective plasmin lysis, could be due to conformational aspects of fibrinogen rather than to structural defects.