Serol. tests are important for understanding the physiopathol. and following the evolution of the Covid-19 pandemic. Assays based on flow cytometry (FACS) of tissue culture cells expressing the spike (S) protein of SARS-CoV-2 have repeatedly proven to perform slightly better than the plate-based assays ELISA and CLIA (chemiluminescent immunoassay), and markedly better than lateral flow immunoassays (LFIA). We describe an optimized and very simple FACS assay based on staining a mix of two Jurkat cell lines, expressing either high levels of the S protein (Jurkat-S) or a fluorescent protein (Jurkat-R expressing m-Cherry, or Jurkat-G, expressing GFP, which serve as an internal neg. control). The Jurkat-S&R-flow test has a much broader dynamic range than a com. ELISA test and performs at least as well in terms of sensitivity and specificity. Also, it is more sensitive and quant. than the hemagglutination-based test HAT, which we described recently. The Jurkat-flow test requires only a few microliters of blood; thus, it can be used to quantify various Ig isotypes in capillary blood collected from a finger prick. It can be used also to evaluate serol. responses in mice, hamsters, cats, and dogs. FACS tests offer a very attractive solution for laboratories with access to tissue culture and flow cytometry who want to monitor serol. responses in humans or in animals, and how these relate to susceptibility to infection, or re-infection, by the virus, and to protection against COVID-19.