Immunochemical assays represent a promising tool for quantification of immunosuppressants in organ transplanted patients, because they require small sample volumes and minimum sample pre-treatment; nevertheless considerations about method specificity, sensitivity and reproducibility cannot be overlooked. The present paper investigates the reliability of using the immunoparticle enzyme immunoassay (MEIA) for the quantification of blood rapamycin (RAPA) levels in therapeutic drug monitoring of renal transplanted patients with respect to a validated liquid chromatography tandem mass spectrometric (LC/ESI-MSMS) method, used as reference. Linearity of MEIA was tested over the range 0.0-30.0 ng/mL, with accuracy and precision within acceptable limits. Fifty-two blood samples were collected from 42 renal transplanted patients and analyzed simultaneously by both methods. The Pearson's regression analysis gave the following parameters: correlation equation [RAPA](MEIA) = 1.330 + 0.776 [RAPA](LC/ESI-MSMS), r = 0.8526, SD = 1.778, p < 0.0001. The obtained average rapamycin concentration was 8.8 +/- 3.4 ng/mL using MEIA and 9.6 +/- 3.7 ng/mL for LC/ESI-MSMS, with an overall underestimation of about 6% of the immunoenzymatic test. Accuracy of MEIA ranged from -33% to 36% with respect to the reference mass spectrometric method. Although immunoenzymatic test represents a fast and sufficiently accurate method for its use in clinical practice, specificity of the assay is still not sufficiently investigated and reference methods and/or Proficiency Testing Scheme should be used as external control.