The photometric method of Fickenscher et al. for the determination of factor XIII (FXIII) activity has been used in the study of 35 patients with severe chronic hepatopathy, in comparison with 25 normal subjects. The FXIII proteic fractions a and b were determined by quantitative immuno-electrophoresis after Laurell. The plasmatic FXIII activity, as well as the proteic fractions a and b, were significantly reduced in hepatopatic patients, in comparison to controls, and proportional to the prolongation of prothrombin times. Ratios between functional and immunological levels of FXIII in hepatopatics were similar to those observed in controls. These results confirm the involvement of fibrin stabilization deficiency in the coagulation defect of severe chronic hepatopathies. The correlations between functional and antigenic values are in agreement with the hepatic origin of FXIII. The method of Fickenscher has been proved to be rapid and simple, and it may be useful in the routine study of hepatopathies, for a better knowledge of the role of FXIII deficiency in the complex coagulopathy of liver diseases, as well as of other acquired FXIII deficiencies.