Recent cross-reactivity studies have evaluated SARS-CoV-1 neutralising antibodies that bind to the RBD-containing S1 subunit. Although both SARS-CoV-1 and SARS-CoV-2 use ACE2 as a receptor for viral entry [3,16], several SARS-CoV-1 RBD-directed mAbs did not cross-react with SARS-CoV-2 RBD [28,29]. Interestingly, CR3022, which was isolated from a SARS convalescent patient [22], showed cross-reactivity to SARS-CoV-2 RBD and recognises an epitope that does not overlap with the ACE2 binding site [28]. Among the four mAbs tested in this study, indirect ELISA showed that 1A9 binds strongest to the S protein of SARS-CoV-2 (Figure 3B). To determine if 1A9 is useful for detection of S protein in a sandwich ELISA, it was paired with CR3022 since 1A9 binds to S2 subunit while CR3022 binds to S1 subunit. As would be expected, these two antibodies can be paired to detect S protein from 15.6 ng/mL (Figure 4A). In addition, mAb 1A9 stained a considerable number of SARS-CoV-2-infected cells at 24 hours post-infection showing that it is sensitive enough to detect the expression of S protein during infection (Figure 4B). Thus, mAbs 1A9 will be useful for studying the kinetics of SARS-CoV-2 replication in vitro and development of diagnostic assays for COVID-19. It is noteworthy that cytotoxic T-lymphocyte (CTL) epitopes also reside at residues 884–891 and 1116–1123 within the S2 subunit of SARS-CoV-1 [30]. Interestingly, the latter CTL epitope overlaps with the epitope recognised by mAb 1A9 [21]. Hence, the S2 subunit may serve as an important antigen for inducing both humoral as well as cell-mediated immunity against SARS-CoV-1 and SARS-CoV-2.