Bovine serum albumin (BSA) is one of the most widely used protein reagents in the scientific community, especially for surface passivation ("blocking") applications in various bioassays. Numerous BSA protein options are commercially available, however, there is scarce information about which ones are preferable for blocking applications.Herein, we conducted biophysical and bioassay measurements to quantitatively compare the conformational, adsorption, and blocking properties of BSA protein reagents that were obtained through six purification methods. Depending on the method, there were significant differences in the conformational and adsorption properties of BSA proteins, mainly due to the presence of fatty acid stabilizers. In turn, we discovered that fatty acidfree BSA proteins exhibit superior blocking performance to fatty acid-stabilized BSA proteins in surface-and nanoparticle-based bioassays. We critically discuss mechanistic factors behind these performance variations and our findings offer a practical framework to guide BSA selection for blocking applications.A common element of bioassays is the need to minimize nonspecific binding of biomolecules in order to maximize assay specificity and sensitivity 1, 2 . To meet this objective, a wide range of reagents, ranging from crude materials such as milk proteins to synthetic nanomaterials such as finely tuned polymers, have been developed to coat target surfaces and to minimize nonspecific binding events-a function that is commonly referred to as