Following our released protocol (Salem et al., 2019a), the rescued phages were purified and exposed to six rounds of panning (three against the rRBD, and three against the inactivated SARS-CoV-2 strain hCoV-19/Egypt/NRC-03/2020, kindly provided by the Center of Scientific Excellence for Influenza Virus, Environmental Research Division, National Research Centre). rRBD was proved genuinely antigenic and immunogenic, exhibiting specific reactivity to anti-SARS-CoV-2 antibody in an indirect enzyme-linked immunosorbent assay (ELISA), and inducing strong seroconversion in immunized mice. The scFv phage display library against rRBD was successfully constructed, revealing 90 % recombination frequency, and great enriching factor reaching 88 % and 25 %25 % in polyclonal Ab-based and MAb-based ELISAs, respectively. Typically, three unique scFvs were generated, selected, purified and molecularly identified. That was manifested by their: accurate structure, close relation to the mouse immunoglobulin (Ig) superfamily, right anchored six complementarily-determining regions (CDRs) as three within variable heavy (vH) and variable light (vL) regions each, and proper configuration of the three-dimensional (3D) structure. Besides, their expression downstream in a non-suppressive amber codon ofE. colistrain SS32 created a distinct protein band at an apparent molecular weight of 27KDa. Moreover, the purified scFvs showed authentic dBET1 immunoreactivity and specificity to both rRBD and SARS-CoV-2 in western blot and ELISA. Accordingly, these developed scFvs platform might be a functional candidate for research, inexpensive diagnostics and therapeutics, mitigating spread of COVID-19. Keywords:SARS-CoV-2, Murine scFv antibodies, Recombinant RBD, Baculovirus expression vector system, Phage display, COVID-19 == 1. Introduction == Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is the causal agent of an acute respiratory disease termed COVID-19. It has been dramatically spread over the world due to its easy way of transmission between persons, resulting in millions of infected and dead cases. This makes a necessity for developing a fast detection test to assist in controlling of outbreaks (Nardell and Nathavitharana, 2020). SARS-CoV-2 is a positive-sense single-stranded RNA enveloped virus with a genome size of 30Kb. It was recognized as a related dBET1 clade to the prototype human and bat coronaviruses of the species SARS-related coronavirus, genusBetacoronavirus, subfamilyOrthocoronavirinae,within the familyCornaviridae(Zhou et al., 2020). One of the SARS-CoV-2 structural components is the spike (S) protein, which is found on the surface and performs an important role in viral entrance into the host cells(Li, 2012,2016). Particularly, the receptor binding domain (RBD) of this protein is influences in infectivity (Wang et al., 2020), its amino acid sequences are frequently divergent that determine the viral host specificity, and facilitating attachment to the targeted host receptor (Li et al., 2005). Moreover, antibodies against this RBD can potentially neutralize the virus, as it encloses the major neutralizing epitopes (Hoffmann et al., 2020). Thus, RBD is a core determinant for tissue tropism, host range and immunogenicity of coronaviruses (He et al., 2006;Du et al., 2009). The main diagnostic procedure for detecting SARS-CoV-2 genomic RNA is the reverse transcription polymerase chain reaction (RT-PCR) (Wang et al., 2020;Corman et al., 2020). Because of the obstacles of RT-PCR, such as the need for a well-equipped laboratory and high reagent costs (Li et al., 2020), serological detection of viral proteins is a potential alternative. However, availability of specific antibodies against SARS-CoV-2 proteins is a key facet to carry out such serological assays (Lee et al., 2008,2010;Dutta et al., 2008). Monoclonal antibodies (MAbs) have been accepted as the foremost element in virus detection, plus they are quickly gaining dBET1 popularity as a supplemental treatment to vaccinations (Jin et al., 2017;Renn et al., 2020;Zhou and Zhao, 2020;Jiang et al., 2020). Constructing an antibody library through phage display technology provides a functional approach for obtaining recombinant MAbs (rMAbs), as the traditional technique for MAbs production is costly and time-consuming (Schofield et al., 2000;Dai et al., 2003;Hu et al., 2003). This method utilizes recombinant DNA technology to facilitate selection of rMAbs and their coding genes (Wittrup, 1999). Furthermore, it could be used to create a therapeutic antibody cocktail of MAbs against a variety of epitopes and might endorse resistance to variant mutations, where SARS-CoV-2 can quickly evolve versions with mutations that circumvent immune responses (Wang et al., 2018;Noy-Porat et al., 2020;Baum et al., 2020;Zost et al., 2020). Single-chain fragment variable (scFv) is one of the most beneficial rMAbs; it can be easily selected through displaying on IL6R a filamentous phage (Moghaddam et al., 2003). A number.