Clinical complications of Extracorporeal Circulation (ECC) are the consequence of a systemic activation of cellular and humoral factors resulting in a dysregulation of the microcirculation. Previously we developed an animal model to study effects of ECC on the microcirculation in vivo. To prevent the systemic inflammatory reaction (SIRS) seen after ECC, colloids were used as priming solution. Intravital fluorescence microscopy was used on the hamster skinfold chamber model. ECC was introduced and the ECC-tube system was flushed with Ringer solution (control), with Dextran 60 (group I) or HES 10% (group II). ECC for 30 minutes resulted in an increase in rolling and adherent leukocytes in postcapillary venules (Roller: 11 +/- 3% to 38% +/- 20% 4 h after ECC, p < 0.05, Sticker: 19 +/- 16 cells/mm2 to 215 +/- 145 cells/mm2 4 h after ECC, p < 0.05; n = 7). Use of Dextran prevented L/E cell interaction (10 +/- 5%; 63 +/- 40 cells/mm2 at 4 h), whereas HES affected only adherent white cells. L/E cell interaction in the microcirculation is an indicator of the systemic activation induced by ECC. Dextran 60 prevented L/E cell interaction without side effects and may be preferable for ECC in regard to inhibition of SIRS.