According to the FDA, if a drug product contains a drug substance that is chemically identical and is delivered to the site of action at the same rate and extent as another drug product then it is equivalent and can be substituted (switchable) for that drug product. Methods used to define bioequivalence as stated by the FDA rules (FDA 21 CFR 320, 24) are: 1) Pharmacokinetic (PK) Studies in healthy volunteers, 2) Comparative clinical trials and 3) Pharmacodynamic (PD) studies (Bioactivity). We have evaluated the switchability of Equoral (IVAX-USA) with Neoral (Novartis Switzerland using all of the above FDA rules. 1) PK; In a Single oral dose comparative, bioavailability study of Equoral and Neoral (Novartis, USA). The pharmacokinetics of Equoral and Neoral were determined with blood levels at: 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3, 3.5, 4,5, 6, 8, 10, 12, 16, 24, 30, 36, 42 and 48 hours. The area under curve (AUC), AUC extrapolated to infinity (AUC0-inf), rate of absorption (Tmax), extent of absorption (Cmax), half time (T1/2) of Equoral and Neoral were all within the 90 % confidence interval of 80%-125% boundaries; 2) Clinical trials: A comparative Clinical Trials The pharmacokinetic of EquoralÒ vs. NeoralÒ in stable renal transplant patients: A multi national multi-center study. 70 patients (22 females and 48 males) with a mean age of 33 years (26-43) were enrolled and have completed the study. Countries are Turkey, Lebanon and Pakistan. Centers are: Baskent University hospital, SIUT (Dow Medical Collage), Rizk hospital and Rayak hospital. In this study the ratios of LSM and the 90 % confidence intervals for the In-transformed Parameters (AUC o-t, AUC inf, Tmax and Cmax) of Equoral and Neoral SGC were 98 % & 95 % respectively which is within the 80-125 % FDA acceptance range. 3) Bioactivity: For immunosuppressive drugs the site of action is the lymphocyte and the measurable response is the decrease in lymphocyte count caused by the relative concentration of the drug in the lymphocyte. In a controlled switch fixed dose study both Equoral and Neoral achieved the same concentration in the lymphocytes and caused the same degree of lymphocyte count reduction. The Results of the testing (Bioavailability- Bioequivalence, Clinical Studies and Pharmacodynamic-Bioactivity) required by FDA for interchangeability “Switchability" of immunosuppressive agents proves that Neoral and Equoral are switchable with each other.