Platelets are known to interact with the surface of synthetic grafts. In the present study we have investigated another blood constituent, the white blood cell, to evaluate the contribution of leucocyte adhesion onto synthetic vascular graft surfaces. Furthermore the effect of low molecular weight dextran (LMWD) was evaluated in vivo. Polytetrafluoroethylene (PTFE) grafts were interpositioned in the femoral circulation in 12 healthy adult sheep. Autologous leucocytes were labelled with Indium-111-oxine and re-injected i.v. after purity and functional evaluation. Graft uptake of 111In-labelled leucocytes were studied continuously for 2.5 hours and two consecutive days using a gamma camera. Six animals received LMWD, 15 ml/h, i.v. infusion during the first day while the other six animals were infused with normal saline in the same amount and rate (controls). The labelled leucocytes showed normal phagocytosis of bacterias and the leucocyte purity was 74 +/- 6%. There was a rapid increase in graft activity initially. In control animals a continuous increase of graft activity was observed throughout the experiment. In the LMWD-treated animals graft activity remained on a steady level after the initial built up of activity and after 2.5 hours there was a highly significant difference between the groups, p less than 0.001. These differences were confirmed by in vitro activity measurement, autoradiography and histological examination of the grafts at the end of the experiment. It was concluded that leucocyte adhesion onto the surface of PTFE grafts occur during the early period after implantation and could therefore be a contributing factor in the thrombogenesis. Administration of LMWD seems to have a beneficial effect since less leucocyte adhesion occurred in those animals treated with LMWD.