To identify the most disparate tacrolimus generic drug formulations among those currently approved in the United States. We will conduct systematic dissolution testing of the brand and all currently approved tacrolimus drug formulations using the FDA-recommended dissolution method. We propose to test and compare the 1mg capsule strength (3 production lots/ manufacturer). In addition, we will compare the different formulations in terms of potency, purity and other quality attributes. This work will be carried out in the GMP-compliant facilities of The University of Iowa Pharmaceuticals and the University of Colorado. Based on these studies the two most disparate generic tacrolimus formulations (Generic Hi and Generic Lo) will be selected and compared with the Brand (PrografR, Astellas, Deerfield, IL) in the clinical trial.