Tacrolimus (TAC) causes a decrease in T cell proliferation by inhibiting the enzyme calcineurine which leads to a decrease in IL2 production. Mycophenolate Mofetil (MMF) is a potent inhibitor of inosine monophosphate dehydrogenase, an enzyme critical for the de- novo synthesis of purines leading to a decreased production of glycoproteins, adhesion molecules and the proliferation lymphocytes and of smooth muscle. In order to determine the bioactivity of TAC and MMF we have developed an assay to determine the level of both Tacrolimus and MMF in the lymphocyte of transplant patients. The levels obtained were correlated with lymphocyte count. Whole blood samples from patients on Tacrolimus and or MMF were collected in EDTA tubes. Immediately the lymphocytes from 2 ml of blood were separated using 1.5 ml of Ficoll gradient, by centrifugation for 30 minutes at 2500 RPM. The lymphocytes were washed 3 times with PBS and the pellet was suspended in 150 UL of PBS and lysed with lysing /precipitation reagent. The lymphocytes cytoplasmic TAC or MMF concentrations were measured using the kits supplied from (ABBOTT) for TAC and (Dade-Behring) for MMF.A corresponding whole blood sample from each patient was used to measure blood levels for TAC and MMF. To determine the level per lymphocytes the value obtained was divided by the number of lymphocytes and is expressed as Pg/Cell. A pharmacokinetic profile for both blood and lymphocytes was constructed for each patient using data corresponding to T0, T1 and T2. The lymphocytes enumeration for T0, T1 and T2, was performed using the Flow Cytometer FACS Calibur from (Becton Dickinson). The average TAC dose was 7.64 mg/day with a T0 of 10.54 WBC of 7797 and a lymphocyte count of 1657. There was no correlation between the dose, T0 level and the lymphocyte count, dose and to (r2= o.0089 and 0.1281 respectively). Similarly there was no correlation between T1, T2 and lymphocyte count (r2= 0.02 and 0.19 respectively. However there was a strong correlation between the B/Cell and the lymphocyte count (r2=0.7349). The higher the concentration of the drug the lower the lymphocyte count. The assay is sensitive to within 0.5 pg/cell, reproducible with a c.v of 5.4 for within assay and 7 for intra assay. We are currently correlating these data with the observed clinical condition and biopsy proven rejections and toxicity.