Ten mongrel dogs underwent carotid artery bypass 17 times with dacron grafts, 4mm in inner diameter and 5cm in length. The grafts in experimental group A were immediately seeded with autologous endothelial cells harvested by 0.25% trypsin solution (n = 7). The grafts in experimental group B were seeded with autologous endothelial cells cryopreserved in liquid nitrogen for 1 week (n = 3). The grafts in contrast group was not seeded with any endothelial cells implanted in the contralateral carotid arteries in group A (n = 7). The number of endothelial cells harvested in the two groups was 0.9 +/- 0.3 x 10. Factor VIII related antigen stain method confirmed the cells harvested by 0.25% trypsin solution to be endothelial cells. The grafts were removed and studied at the end of 2, 4, and 6 week after implantation. The total patency rate of group A was 85.7% (6/7), and that of the contrast group 57% (4/7). The weight of thrombus in group A was 40.6 +/- 36.9mg and the contrast group 85.9 +/- 26.3mg (P < 0.01). Scanning electron microscopy revealed that the cells at the middle segment of the grafts in the group A and B had the characteristics of ellipse, and tight intercellular junction, while the middle segment grafts in contrast group were covered by platelets, erythrocytes, leukocytes and fibrin. Transmission electron microscopy, factor VIII related antigen stain method, and vimentin stain method demonstrated that the cells in the group A and B showed characteristics of endothelial cells. It was concluded that endothelialization of the dacron grafts could be accelerated and the quality could be improved by immediate seeding with endothelial cells or by seeding with endothelial cells cryopreserved in liquid nitrogen for 1 week.