Tacrolimus ( FK506 ) is a potent immunosuppressive drug used for prevention of rejection following transplantation. Several methods including immunoassays have been used for monitoring tacrolimus levels. The purpose of this study was to compare the effects of some hematological parameters such as hematocrit (Htc), hemoglobin (Hb), red blood cell (RBC), mean cell volume (MCV), mean cell hemoglobin (MCH), mean cell hemoglobin concentration (MCHC), red cell distribution width (RDW) and platelet (PLT) counts on whole blood tacrolimus concentrations which were measured by two different analytical methods namely, the microparticle enzyme immunoassay (MEIA II) and enzyme multiplied immunoassay technique (EMIT). The tacrolimus results (n=2430) obtained with two methods [EMIT(n=1171), MEIA II (n=1259)] and some hematological parameters have been compared in kidney, liver and combined kidney- pancreas transplant patients (n=162) during a two year period following transplantation. Our results shown that the whole blood tacrolimus concentrations which were measured by MEIA II method were much more significantly affected by hematological parameters than those by EMIT method. We also found that in MEIA II method, the RDW (r=0.479, p < 0.01) showed a stronger significant correlation with tacrolimus level than hematocrit (r=-0.239, p < 0.01) in all patients. Negative significant correlation was also observed between the hematocrit level (r=-0.468, p< 0.01) and tacrolimus concentrations when the patients have got a hematocrit value below the 25% in MEIA II method. Evaluation of hematologic factors that influence the whole blood concentrations of tacrolimus may help for drug dosage decisions. We emphasize that EMIT method might be preferred to MEIA II method in determination of tacrolimus levels in anemic patients after transplantation.