This guidance is intended to provide recommendations for study sponsors and/or drug manufacturers, contract research organizations (CROs), site management organizations (SMOs), clinical investigators, and independent third parties regarding the procedure for handling reserve samples from relevant bioavailability (BA) and bioequivalence (BE) studies, as required by 21 CFR 320.38 and 320.63. The guidance highlights (1) how the test article and reference standard for BA and BE studies should be distributed to the testing facilities, (2) how testing facilities should randomly select samples for testing and material to maintain as reserve samples, and (3) how the reserve samples should be retained. The guidance also clarifies and emphasizes points addressed in §§ 320.38 and 320.63.