This guidance is intended to help manufacturers of positron emission tomography (PET) drugs meet the requirements for the Agency’s current good manufacturing practice (CGMP) regulations for PET drugs (21 CFR part 212). Most PET drugs are designed for parenteral administration and are produced by aseptic processing. The goal of aseptic processing is to make a product that is free of microorganisms and toxic microbial byproducts, such as bacterial endotoxins. A media fill is the performance of an aseptic manufacturing procedure using a sterile microbiological growth medium, in place of the drug solution, to test whether the aseptic procedures are adequate to prevent contamination during actual drug production. Media fill procedures recommended in this guidance apply only to sterile PET drugs manufactured by aseptic processes under 21 CFR part 212.