The specific aim is to conduct a randomized, single dose, four-treatment, four-period cross-over bioequivalence (BE) study in 24 healthy normal adult volunteers (males and non-gravid females) to evaluate the in vivo performance of fresh and aged brand name and generic amorphous solid dispersion (ASD) preparations of tacrolimus. The pharmacokinetics of tacrolimus as fresh Prograf® and aged Prograf®, fresh generic tacrolimus capsules and aged generic tacrolimus capsule will be determined and compared in healthy volunteers. The hypothesis to be tested is that the tacrolimus in the amorphous solid dispersion formulation will be partially crystalized upon treatment with controlled heat and humidity and will demonstrate lower absorption (lower relative bioavailability) compared to the fresh product. Moreover, the expectation is that the heat- and humidity-stressed generic formulations will not be robust to crystallization as the stressed brand name drug and will demonstrate a decreased in vivo performance and loss of bioequivalence.