Double-stranded nucleic acid derivatives having RNA as the parent body, are characterized in that the total molecular size distribution of the derivatives falls within the range of from 4S to 13S expressed as their sedimentation constant. The derivatives may be further characterized in that the molecules for the maximum distribution in their total molecular size distribution have a base number of from 50 to 10,000. The derivatives may be used as active ingredients in antitumor or antiviral compositions.
