The Spike S1 (B.1.617.2.1; Delta Plus Variant) (SARS-CoV-2): ACE2 TR-FRET Assay is designed to measure the inhibition of the binding between SARS-CoV-2 Spike S1 (B.1.617.2.1; Delta Plus Variant) and human ACE2 in a homogeneous 384 reaction format. This TR-FRET-based assay requires no time-consuming washing steps, making it especially suitable for high throughput screening applications. The assay procedure is straightforward and simple; the test inhibitor compound is incubated with biotinylated Spike S1, Eu-labeled ACE2, and dye-labeled acceptor for one hour. Then the TR-FRET signal is measured using a fluorescence reader capable of measuring Time Resolved-Fluorescence Resonance Energy Transfer (TR-FRET).