Vascular prostheses made of polyester or polytetrafluoroethylene fibres are most frequently used nowadays. Woven polyester prostheses are sealed following the suturing in the circulatory system, but they cause a tissue reaction. Knitted polyesters grafts cause a lesser tissue reaction, but demand intraoperative sealing. Thus numerous tests to improve them are conducted. TRICOMED S.A. (Poland) has modified the company's own double velour knitted vascular prosthesis. The graft's physical and chemical features have been changed and hydrophilia has been used, due to which dripping the graft takes little blood and its tightness is achieved in a dozen or so seconds. The evaluation of surgical qualities as well as the healing-in process of the modified knitted hydrophilic prosthesis DALLON H has been conducted at the Department of Experimental Surgery and Biomaterials of Wrocław Medical University. DALLON H prosthesis was compared to DALLON, a standard double velour knitted prosthesis. The tests were performed on 16 Polish breed piglets with 25-35 kg of weight and 2.5 months of age. Two follow-up periods of 3 and 6 months were established following the surgeries. In each group six piglets had a 6-cm-long segment of DALLON H prosthesis sutured in the thoracic aorta defect, whereas two piglets received a 6-cm-long segment of DALLON prosthesis. The evaluation of surgical qualities of both kinds of grafts was performed intraoperatively. The main emphasis was put on the conditions in which the grafts became tight and the way they were handled during the aortal anastomosis. After 3 and 6 months the animals were autopsied. A macroscopic analysis of the incurred preparations was performed. Roent-genograms of the thoracic aorta sections replaced by the grafts were made in order to establish the shape of their lumen and present calcium salt deposits in their region. Light microscopic tests were performed by dying the preparations with hematoxiline and eosine to differentiate tissues and basic morphotic elements; using the van Giesen method, in order to reveal collagenous fibres; using the Weigert method to reveal elastic fibres, and using the Koess method to reveal calcium salt deposits. An ultrastructural evaluation of the internal surface of the grafted vascular sections was performed by a scanning electron microscope. In view of the experimental tests performed it has been established that DALLON H prosthesis used in vascular surgical procedures is not difficult and the prosthesis can be handled in the same way as some other knitted vascular prostheses. DALLON H does not need preclotting and when the blood flow has been restored through the grafted section the prosthesis immediately becomes completely tight. In view of the tests performed no differences in the healing-in process between the modified DALLON H and DALLON prostheses were observed.