One of the drugs that have the prospect of being used for the therapy of COVID-19 is the modified guanine analogue - favipiravir. Favipiravir (T-705; Avigan®) is an antiviral agent developed by Fujifilm Holdings and approved in 2014 in Japan for the treatment of influenza. Favipiravir can effectively inhibit the RNA transcriptase of RNA viruses such as influenza virus, Ebola virus, yellow fever virus, enteroviruses, and chikungunya virus. It has recently been shown to be active against coronavirus in a transplantable monkey kidney cell line "Vero E6". Thus, favipiravir is one of the real contenders for a drug for possible use in the treatment of patients suffering from COVID-19. Favipiravir has been approved in Russia, India, China and Japan for the treatment of COVID-19, and it has been reported that favipiravir led to faster recovery in mild to moderate patients. At the same time, a common side effect was discovered, such as high levels of urea in the blood. This increase in urea is especially dangerous for patients with kidney problems. In addition, it is contraindicated in pregnant and lactating women. It seems to us that these side effects of favipiravir can be avoided and, at the same time, the effectiveness of this agent can be increased if its molecule is transformed into ribonucleoside (ribofavipiravir), a compound more reminiscent of the active prodrug form of the ribofavipiravir-5′triphosphate [1]. This process of transformation of favipiravir into its ribonucleoside was carried out using the previously obtained recombinant purine nucleoside phosphorylase (PNPase) of Escherichia coli, according to the scheme:schemaThe reaction mixture (10 ml) containing 10 mM guanosine, 5 mM favipiravir, 0.15 mM Kphosphate buffer (pH 7.0), and PNPase was incubated for 4 h at 50 °C. The accumulation of the product was recorded by thin-layer chromatography on Silufol-UV254 plates (Merck, Germany) in the solvent system isopropanol-chloroform-25% ammonia (10:10:1 v/v). The rate of transformation of favipiravir into its ribonucleoside reached 43 % in 4 h of the reaction.