Renal transplantation (KTx) patients receiving calcineurin inhibitors-cyclosporine (CsA) or tacrolimus (TAC)-are known to be at risk for the development of posttransplant hemolytic uremic syndrome. The syndrome has been reported to occur more often with CsA than TAC treatment. It has also been noted that CsA affects erythrocyte membrane fluidity. The aim of this study was to investigate whether the impairment of erythrocyte membrane fluidity is similar among patients under treatment with different calcineurin inhibitors (CsA or TAC). Venous blood was collected from 29 KTx patients and from nine healthy volunteers. To investigate the fluidity of intact erythrocyte membranes spin label electron paramagnetic resonance spectroscopy was applied. Comparison of values for controls versus CsA-treated patients demonstrated a significant decrease in membrane viscosity in the hydrophobic region of the lipid bilayer among CsA-treated patients, whereas there was no significant difference between control and patients treated with TAC. There was no significant difference in the molecular order close to the polar heads of lipid molecules among all groups. The observed changes in erythrocyte membrane fluidity among CsA-treated patients and the lack of this phenomenon in the TAC group may correlate with more frequent prevalence of hemolytic anemia among CsA than TAC-treated patients.