57 patients who had undergone surgical operations because of varicosis were examined for the influence of the mode of anaesthesia on the plasma levels of factor XIII. Within a randomized study the patients were given by alternate order halothane (NA) (n = 19), neurolept-analgesia (NLA) (n = 19) or peridural anesthesia (PA) (n = 19). Pre- and postoperatively as well as at the 1st, 3rd, 5th, and 7th p.o. day the plasma levels of factor XIII, fibrinogen (factor I) and fibrinogen-degradation-products (FDP) were measured. FDP were not found. The levels of factor XIII remained within the normal range. Fibrinogen showed an identical course: the postoperative findings were lower than the initial ones. 24 h later the levels were higher. At the 7th p.o. day the most important readings were found. In comparing the three methods of anaesthesia the most obvious differences in the data on factor XIII were in the peridural anesthesia-group, especially on the 3rd p.o. day with significant lower results than in the other groups. The modes of anaesthesia used had no visible influence on the range of factor XIII.