Activated leukocytes are known to generate reactive oxygen species which may attack other cells in their surroundings. On the other hand, rheological properties of red blood cells (RBC) are affected by oxidant attack.This study was designed to determine the influence of activated leukocytes on RBC rheology in vitro. Activated leukocytes were isolated from the Guinea pigs with an induced E. Coli sepsis and incubated with RBC obtained from normal animals. After two hours of incubation with activated leukocytes, RBC deformability indices were increased 1.23 +/- 0.05 (Mean+/-Standart error) times (compared to the control RBC suspensions) while incubation with leukocytes obtained from healthy animals increased deformability indices 1.12 +/- 0.03 times. Thiobarbituric acid reactive substances as a measure of lipid peroxidation were increased 2.95 +/- 0.36 times in the RBC suspensions incubated with activated leucocytes, but 1.70 +/- 0.10 times in suspensions incubated with normal leukocytes. These results suggest that RBC mechanical properties might be affected by activated leukocytes and possibly an oxidative mechanism plays role in this interaction.