Nucleoside compounds of the formula ##STR1## wherein: B is a purine or a pyrimidine; X and X' are H, OH or F, provided that at least one is H; Y and Y' are H, OH, OCH.sub.3 or F, provided that at least one is H; Y' and Z together form a cyclic phosphate ester, provided that Y is H; or Z is ##STR2## where n is zero, one, two or three; and Z' is N.sub.3 or OCH.sub.3 ; provided that when X' and Y' are OH and Z' is N.sub.3, B is not cytosine, and when X' and Y' are OH and Z' is OCH.sub.3, B is not uracil, adenine or cytosine; and the pharmaceutically acceptable esters, ethers and salts thereof, have been found to have potent antiviral activity with a high therapeutic ratio.