The level of factor XIII circulating in the blood plasma was determined with a commercially available screening method of the Behringwerke, Marburg, Germany. In a randomized study, the blood of 76 of both sexes and different ages was analyzed before and after surgery. In most extensive laparotomies, factor XIII declined as an expression of the vascular processes during and after the operation. The same effect was demonstrated after surgical excision of stomach or of colon carcinomas. Concurrent with the factor XIII level, fibrinogen concentration was also reduced. While the replenishment of factor XIII extended over a period of several days postoperatively, the fibrinogen synthesis was increased and exceeded normal levels after 2 to 3 days. Inspite the operational activities, factor XIII was never so low that a substitution was indicated.