Neutralizing monoclonal antibodies (mAbs) being developed to combat COVID-19 are generated against the receptor-binding domain (RBD) of the spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The anti-RBD mAbs prevent binding of the S protein to its cognate receptor, angiotensin-converting enzyme 2 (ACE2), on target host cells. Three neutralizing mAb regimens have been given emergency use authorization for treatment of COVID-19. (1) Casirivimab and imdevimab bind distinct epitopes on the RBD with dissociation constants KD of 46 and 47 pM, respectively. Imdevimab binds the S protein RBD from the front or lower-left side, while casirivimab targets the spike-like loop from the top direction (overlapping with the ACE2-binding site3,68). (2) Bamlanivimab binds an epitope on the RBD in both its open confirmation and its closed confirmation with dissociation constant KD = 71pM, covering 7 of the approximately 25 side chains observed to form contact with ACE2 (ref.4). (3) Bamlanivimab and etesevimab bind to distinct, but overlapping, epitopes within the RBD of the S protein of SARS-CoV-2. Etesevimab binds the up/active conformation of the RBD with dissociation constant KD = 6.45 nM (ref.5); it contains the LALA mutation in the Fc region, resulting in null effector function.