PURPOSE: To provide the subject compounds showing an antiviral effect and a cell-proliferation inhibitory effect.

CONSTITUTION: Derivatives of formula I (X is H, a lower alkyl, etc.; R1 and R2 are H or an OH-protecting group) and salt thereof, e.g. a compound of formula I in which X=H and R1=R2=isopropylidene. The compound of formula I can be obtained, e.g. by the following method; A compound of formula III is obtained from uridine of formula II as the raw material and an amine is reacted therewith at 40-70°C for 10-50hr. An OH-protecting group is subsequently introduced to 5'-OH and nitric acid, etc., is then reacted therewith. Reflux in toluene is subsequently carried out at 100-200°C to obtain a compound of formula IV and the 5'-OH-protecting group of the compound of formula IV is eliminated. As necessary, 2'- and/or 3'-OH-protecting group is also eliminated to obtain an isoxanthosine derivative of formula V and a tosylation agent is reacted therewith in a basic solvent at 0-30°C for 1-50hr. After concentration, the resultant residue is treated with an alkali.