A pure (-)-9-[1'S, 2'R-bis(hydroxymethyl)cyclopropan-1'-yl]methylguanine is prepared by:suspending or dissolving a mixture comprising (-)-9-[1'S, 2'R-bis(hydroxymethyl)cyclopropan-1'-yl]methylguanine and (7-[1'S,2'R-bis(hydroxymethyl)cyclopropan-1'-yl]methylguanine in an alcohol or a hydrous alcohol, reacting the resulting suspension or solution with an alkali metal hydroxide or alkoxide, and selectively precipitating crystals of nucleic acid derivatives of formula (I) ##STR1## wherein M represents an alkali metal.